PURPOSE: To accomplish satisfactory fastening to a housing without requiring a special tool in an arbitrary position by providing the housing with a receiving member for receiving a sleeve-shaped additional part in a protection cover, and freely adjusting the diameter of the receiving member.

CONSTITUTION: A housing 1 is provided with a receiving member 5 for receiving a sleeve-shaped additional part 4a in a protection cover 4. The receiving member 5 is formed as a sleeve where a slit is formed, and the circumferential end parts 5a, 5b of the sleeve are advantageously capable of relatively moving to approach and separate to adjust the diameter of the receiving member 5. Such a slit is formed in the sleeve 5, whereby the protection cover 4 having a sleeve-shaped additional part can be surely fastened without any obstacle in any arbitrary rotational position.