It is Ko Ko (Don Ko) from Myanmar.
I visited to the Hakuun caves [白雲洞] is located into the side of Taishaku-kyo Valley in Shobara city and it is long 200 meters.
Although it is very narrow, the route is smooth, well maintained and easy to walk. You can see many stalactites, well-lit, interesting rock formations in the caves which are given quirky names, most of which are translated into passable English.
It takes 20 minutes to cover and all and the entrance fee is very reasonable. The admission fee for adults is 250¥, high school students is 200¥, elementary and JHS is 150¥ and free for kids.
Opening hours for the cave is 09:00-17:00 between March and December, but sometimes it is closed on Mid-December to end of February although sometimes open on weekends and holiday. Early-March to mid-July closed in Thursday.
Please enjoy with Japanese natures.