Nagashibina refers to a festival that was once common throughout Japan but is now only celebrated in a handful of places, Mochigase being one of them.
These are some of the dolls used in the festival. Based, like much of Japanese ancient religion, on Taoist rituals, the dolls are akin to scapegoats, bad luck, impurity, sin, etc being carried away by the dolls as they float down river to the sea.
The festival takes place at the end of March, but if you can't attend it the museum has displays showing the festival, including this tableau with dolls.
There are also life-size tableaux showing Hina Matsuri.
With over 1,000 dolls on display it is easy to spend several hours in the museum.
For some reason I was more drawn to the almost two dimensional paper dolls rather than the more intricate (and expensive) dolls.
If you are interested in dolls I would suspect it is well worth a visit, though like many of the more interesting sites in japan it is nowhere near Tokyo or a Shinkansen station. Mochigase is located on a local rail line south of Tottori City. Entrance to the museum is a mere 300yen.