Abstract: According to one embodiment, an electronic device includes a device body which includes a main system unit and an option unit detachably connected to the main system unit. The main system unit includes a first housing including a first connection side surface and a first engagement portion at the first connection side surface, and a main control circuit board in the first housing. The option unit includes a second housing including a second connection side surface opposing to the first connection side surface of the first housing and a second engagement portion at the second connection side surface, an option device at the second housing, and a lock mechanism configured to lock the second housing, and a release member configured to release from locking.