PURPOSE: To simplify the unit installation of a device and to improve safety by making a unit with a groove stride a shaft provided on a device side and providing a shaft introduction part and a shaft fitting part.

CONSTITUTION: A plurality of units 2, 3 are installed into a device 4 through a unit installation mechanism 7. Namely, the unit installation mechanism 7 comprises a shaft 8 provided in the device 4 and groove 9 provided in the unit 2. The shaft 8 has a core metal 8a covered with a rubber collar 8b and a groove 9 is provided in the wall of a housing of the unit 2 which is formed in a lateral U-shaped so as to cross the shaft 8. The groove 9 is formed in an L-shaped form which comprises a shaft introduction part 9a and a shaft fitting part 9b. The groove width (A) of the shaft introduction part 9a is made larger than the outer diameter of the shaft 8 and the groove (B) of the shaft fitting part 9b is made narrower than the outer diameter of the shaft 8. Thereby, the unit installation is simplified and the safety is improved.