ARITHMETIC DEVICE

PROBLEM TO BE SOLVED: To effectively prevent false connection or damage to a connector in an arithmetic device including a unit housed in an opening and connected thereto by the connector, such as a hard disk.SOLUTION: The device includes: a casing having an opening 265 for housing a unit 270 provided with a unit-side connector 273; and a device-side connector 269 to be connected to the unit-side connector, which is arranged in a position facing the position on the deep side of the casing 265. The casing includes protrusions 266a, 266b for preventing incorrect operation that protrudes from side surfaces of the opening 265 to the inside of the opening. The protrusions are free from interference with the unit 270 when the unit 270 is disposed in a normal attitude in the opening to allow the unit-side connector 273 to be inserted or removed to/from the device-side connector 269, and interferes with the unit 270 when the unit 270 is pressed toward the device-side connector 269 with at least the front side raised.