PURPOSE: To prevent a short circuit from occurring between a sliding piece and terminal contact sections due to the chain movement of abrasion powder and the deterioration in performance by boring one or more holes on the inside bottom between the terminal contact sections.

CONSTITUTION: At least one or more holes 26 are bored on the bottom of a housing 16 between the contact 6a of a terminal 6 and the contact 8a of a terminal 8. When a rotor 44 is rotated, abrasion powder is generated in a chain reaction by the contact friction between a sliding piece 54 provided on the rotor 44 and the contact sections 6a, 8a and 10a of a group of pin terminals 6, 8 and 10 arranged on the inside bottom 20 in the housing 16. The abrasion powder is formed in a chain reaction between the contact section 6a and the contact section 8a, for example, which may cause a short circuit, but the chain state of the abrasion powder is cut off by the holes 26, and the short circuit between the terminals is prevented. Troubles such as a short circuit are thereby eliminated, and the performance can be improved.