Mt. Musa-dake (武佐岳, Musa-dake), a minor peak of the Shiretoko Mountains, sits just south of Mt. Shari-dake (斜⾥里岳, Shari-dake) and is easily visible from Kaiyoudai (開陽台), a large outlook at the north of the vast plains of southeastern Hokkaido. The name of the mountain comes from an Ainu word pronounced mosa, mose, or muse, meaning 'nettle.' The summit affords great views of the Shiretoko Mountains in the northeast, as well as the and Konsen Plains (根釧原野, Konsen-genya). At the bottom of the mountain, you'll find a small hut called Keisei Lodge (憩清荘, Keisei-sou).
To find the trailhead you're going to need some sturdy map skills. Here goes. From central Nakashibetsu Town (中標津市街, Nakashibetsu-shigai), travel down the 24th Street (⼆⼗四線道路, Ni-juu-yon-sen dooro) towards Kaiyoudai. Near Kaiyoudai, take 19th Street North (北⼗九号の道路, Kita juu-kyuu-go-no-dooro) towards the Kutekunbetsu River (クテクンベツ川, Kutekinbetsu-gawa). Past the river you'll see the Tozanguchi-mae Bus Stop (登⼭山⼜⼝口前バス停, lit. "trailhead-front bus stop").
Turn onto the road leading up towards the mountains and turn right at the sign, and you'll shortly come to the Old Trailhead (旧登山口, Kyuu-tozan-guchi), where you'll find a viewing platform and a toilet. Follow the forest road up and easy grade for about 1 kilometer and you'll hit the parking lot with the current trailhead near a bit of a lumber yard.
From here you'll walk the an old lumber road. After about half an hour you'll arrive at Keisei Lodge. The lodge is unmanned but you can fill up on water from the Kei-no-sawa River (憩ノ沢, Kei-no-sawa) here and stay the night. It's a lovely place to stay the night after a short late-day climb up to the lodge.
From the lodge, you'll head up a forested slope up onto a ridgeline covered in Sakhalin fir. From here, looking across the valley, you should see the peak of Musa-dake looming over the treetops.
You'll pass across a wide ridge of Erman's birch and up a short slope before coming out onto Miharashidai (⾒晴台). Though you'll only be at 800 meters above sea level at this point, you'll find yourself in a sea of (typically high-altitude) dwarf stone pine Tschonoski's rhododendron.
As you climb, you'll start seeing other flowers like cowberry and wild rosemary, which should get you in the alpine mood. Continuing along the ridgeline towards the summit, if you look over the far side of the mountain you should spot Shari-dake and the boundless plains at its foot. From here on out it's an easy, enjoyable stroll to the summit. In early summer, the blossoms on the Japanese alpine cherry, creeping dogwood, and Spiraea betulifolia are particularly beautiful alongside the trail.
At the rocky summit of Musa-dake, you'll find a 360-degree view waiting for you. As Musa-dake is somewhat to the side of the main bulk of the Shiretoko Mountains, the view from nearby Shari-dake and Mt. Unabetsu-dake (海別岳, Unabetsu-dake) all the way down to Mt. Rausu-dake (羅⾅岳, Rausu-dake) can be especially breathtaking. Below you, the windbreaks on the fields of Konsen-genya make the wide land look like a patchwork quilt. (And to think, only a few hundred years ago, it was boundless forest.) Out east, if the weather's cooperative, you can also see the Straits of Nemuro (根室海峡, Nemuro-kaikyou), the Kuril Islands of Kunashiri (国後) and Etorofu (択捉), and the graceful arm of Notsuke Peninsula (野付半島, Notsuke-hantou), among other typically East-Hokkaido sights.
Take your time at the summit, and when you're ready, head back down the trail you came up.
For the first half of the season, expect some snow to streak the views of the Shiretoko Mountains -- and during the second half, try to spot the dustings of new snow heralding the coming winter. In June, the leaves on the Erman's birch will seem greener than green against the white branches; by late July you'll preoccupy yourself with the Tschonoski's rhododendron, Kamchatka rhododendron, and Senecio kawakamii. The leaves start to change around mid-September.