To regulate the rotational speed of a calibration rotator easily and accurately over a wide range by providing an AC motor and an inverter circuit built in the power supply circuit thereof.

When the r.p.m. of a tachometer 21 is inspected, driving frequency, i.e., driving r.p.m., of an AC motor 1 is set for an inverter circuit 4 by operating an operating unit 5. When the power supply circuit is closed to start the motor 1, the armature shaft 2 is rotated at a set r.p.m. Rotation of the armature shaft 2 is transmitted through a coupling 12 to the rotary shaft 11 of a rotation detector 6. The rotation detector 6 delivers a rotational signal to a frequency counter 13. Based on the count of counter 13, the motor 1 is controlled through the operating unit 5 such that a preset frequency is attained. A contactor 22 is rotated together and the rotational speed of armature shaft 2 is measured by means of the tachometer 21. When any difference is present between the measurement of tachometer 21 and the set r.p.m. of armature shaft 2, it represents the lower accuracy of tachometer 21.