Seems to be some kind of lookout post over the lake.
See 
- https://commons.wikimedia.org/wiki/File:Lake-toya%E3%80%81%E6%B4%9E%E7%88%BA%E6%B9%96_%E6%B5%AE%E8%A6%8B%E5%A0%82%E5%85%AC%E5%9C%92.jpg
- https://commons.wikimedia.org/wiki/File:Shrine_Toyako_hokkaido.jpg