Probably the most famous donuts store is Dunkin' Donuts but it does not exist in Japan except in US military bases. However, Dunkin's first oversea store opened in Tokyo in 1970 but closed all stores in 1998.
The most famous and sole nationwide donuts store in Japan is Mister Donuts which is actually established in the United States. However, currently there is only one store remains in Illinois. Mister Donuts was acquired by Dunkin' Donuts in 1990 and all other Mister Donuts in US turned to Dunkin' Donuts.
On the other hand, Japanese Mister Donuts is owned and operated by Duskin Co, which is actually not a restaurant company. Duskin's main business is cleaning business.
Mister Donuts also offer pastas and dim sums. Therefore if you are hungry and want to eat savory and sweet foods, Mister Donuts is a good choice in Japan.