We were tipped by An War Sánchez that Casio has a museum, officially called Toshio Kashio memorial museum of invention. All kinds of inventions and Casio products are exhiibited including the (many) musical instruments that Casio developed and marketed, including the Casiotone range keyboards, the synthesizers and the huge COSMO synthesizer as well.
The museum has a website which is in Japanese only, but you can access pages through a Google translation, such as the pages about the museum rooms via this link. The original website can be found here.
You can have a look in the museum through some video’s on the website, which are available on Youtube too, such as this one:
The museum is located at: 4-19-10 Seijo, Setagaya-ku, Tokyo 157-0066 (Japan). Please note that you need to make an appointment via e-mail.