To prevent theft of both of a device main body and an expansion device detachably mounted on the device main body simultaneously.

An HDD 11 having a lock mechanism 18 composed of a cover member 13 on which a first bore 17 is formed, a slide piece 14 which a second bore part 19 is formed on and is slidably operated along the cover member 13 and an operant 15 for operating the slide piece 14 is attached to the device main body 2 as the expansion device. The first bore 17 formed on the cover member 13 of the HDD 11 is formed within the movement range of the slide piece 14. The first bore 17, when the HDD 11 assumes a lock state with respect to the device main body 2, communicates with the second bore 19 to constitute a bore for locking 21, and, when the HDD 11 assumes an unlocked state with respect to the device main body 2, is blocked with the slide piece 20.