Gunma's caldera mountain
Mt. Akagi (1,828 m / 5,997 ft) is one of Gunma's Three Mountains (alongside Mt. Haruna and Mt. Myōgi) and the iconic skyline of southern Gunma. It is not a single peak but a stratovolcano caldera roughly 18 km wide, with two crater lakes — Ōnuma and Konuma — at its centre and a ring of 18 subsidiary peaks around the rim. The highest of these rim peaks is Kurobi-san (1,828 m), and the Hyakumeizan registration officially applies to it. The lakes inside the caldera, the working shrine on the larger lake's shore, and a small alpine wetland called Kakuman-buchi make Mt. Akagi feel more like a high-altitude lake district than a single peak.
The standard climb: a two-peak loop from the lake
The most popular itinerary is a Kurobi-san to Komagatake loop. From the Kurobi trailhead on the shore of Lake Ōnuma at 1,360 m, the trail climbs steeply through subalpine forest to the Kurobi summit in about 80 minutes. From there a gentle ridge walk leads south to Komagatake (1,685 m) and a separate descent trail back to lakeside. Total loop: about 3.5 hours, 5 km, and 470 m of vertical gain. This is one of the most beginner-friendly Hyakumeizan day climbs in Japan, and a sensible 'first 100 Famous Mountain' from Tokyo. Shorter alternatives include Jizō-dake (a separate caldera-rim peak with a paved road to near the summit) for a 2.5-hour loop.
Why this is one of the easier Hyakumeizan to reach
From Tokyo Station, the Jōetsu Shinkansen reaches Takasaki in 50 minutes; a local connection brings you to Maebashi in another 15 minutes. From Maebashi Station, a Kanetsu Kōtsū bus runs to the Akagi Visitor Center at the lake in about 70 minutes — the bus is the limiting factor, with only a few departures per day, so check the schedule before planning. By car, the Kan-Etsu Expressway exits at Akagi or Maebashi IC, with about an hour up the scenic Akagi-Dōro highway to free lakeside parking lots. An early Shinkansen out of Tokyo has you walking by 10 a.m. — comfortably within day-trip range.
Ōnuma Lake, Kakuman-buchi marsh, and Akagi Shrine
The real reward of a visit is not the summit but the caldera floor. Lake Ōnuma (4 km circumference, 16 m deep) freezes solid in winter and becomes one of the most popular smelt-fishing destinations in eastern Japan. On its small offshore islet sits Ōdō Akagi Shrine, a vermilion-lacquered structure connected to the shore by a small bridge — one of the most photographed sacred-building-on-water scenes in northern Kantō. The smaller Kakuman-buchi marsh on the eastern caldera floor is a wooden-boardwalk wetland nicknamed 'Little Oze.' Its 1-hour loop pairs naturally with the Kurobi climb to fill a full day on the mountain.
When to climb
Hiking season runs late April through mid-November. Three windows stand out: early to mid-May for the Akayashio rhododendrons (pink wild azaleas that are the mountain's signature flower), June for the orange Rengetsutsutsuji near the lake, and mid-October to early November for autumn colour. Winter access remains possible by car to the lake, with snow-shoeing on the marsh and ice-fishing on the lake; the summit itself in winter is a real snow climb with strong wind on the rim.
What to pack
Standard sub-2,000 m kit: long-sleeve baselayer, fleece or wind shell, full waterproof rain shell, gloves, beanie in cold months, ankle-supporting hiking boots. The Kurobi descent and Komagatake summit involve tree-root sections and short rocky steps where boots make a real difference over sneakers. Summit temperatures in summer average 15 °C with frequent wind. Carry 1 L of water — the only on-trail source is at the lake-level trailhead.
Mt. Akagi is the source of the famous Akagi-oroshi — a strong, dry, cold north-westerly that flows down off the mountain across the Kantō plain in winter. The whole climatology of southern Gunma in winter is shaped by this wind. If you climb on a cold high-pressure winter morning, you are feeling the source of weather that millions of people 100 km away in Tokyo are experiencing at the same moment.
From Kurobi summit
A short side trail from Kurobi summit leads to a north-facing viewpoint called Kurobi-Ōkami. On a clear day, the view runs from the Tanigawa massif and Niigata border peaks to the north, the Nikko range and Mt. Nantai to the east, Mt. Tsukuba and the Kantō plain to the south, Mt. Fuji on the southwestern horizon, and Mt. Asama and the Shinetsu border to the west. On exceptional winter mornings the Tokyo Skytree is visible 100 km to the south. After descending, the natural follow-up Hyakumeizan are nearby in Gunma: Mt. Haruna to the west, Mt. Myōgi to the southwest (technically harder), and Mt. Tanigawa to the northwest. A four-day 'Three Mountains of Gunma + Tanigawa' trip uses Mt. Akagi as a sensible first day.
