To make it possible to execute a low-cost test with a small power in a short time without danger or restriction by rotating the rotary electrode part engaged with the periphery of a commutator with respect to the commutator by a rotary drive source detachably mounted on the shaft of a rotor.

A rotary electrode part 10 is rotatably driven by a drive gear motor 12 fixed to the collet chuck 11 engaged on the one shaft of a rotor 1. When a rotor 13 is rotated by the motor 12, a main arm 15 and a sub-arm 16 are rotated, and a probe 19 is rotated as remains separate from a commutator 6. At this time, a light is emitted by a groove detecting sensor such as, for example, a photosensor 17 to read the groove of the commutator 6, a predetermined position is decided, and the rotation is stopped there. Then, a linear solenoid 18 is energized to advance a probe 19 with respect to the commutator 6 and brought into contact with it. Thus, information relative to necessary electric characteristics is sent to a measuring unit.