Shidakas Utopia
Apparently this theme park opened in 1965 but closed in or around 2000. The park is located in the mountains outside of Beppu and sits beside an empty roadside.
I squeezed between a gap in a fence beside a small ticket office and then walked along a path. Most of the complex was covered in foliage but there were still a few bits and pieces of its former glory scattered around including some cool retro roller-skates.