PURPOSE: To monitor a rotational frequency properly by providing a sampling signal generating circuit, a battery, and a rotational angle detector for ≤1 turn, and allowing an output circuit to receive outputs of a counter circuit and the rotational angle detector in parallel.

CONSTITUTION: A pulse generated for every turn of a rotating body 1 is utilized to counts up and down the rotational frequency of the rotating body 1, the battery 8 as a power supply to an integrated circuit is provided, and sensors 3 and 4 and amplifiers 5 and 6 are operated with pulses from the sampling signal generating circuit 77. A code plate 10 on the rotating shaft of a motor and detectors 111 and 112 provided opposite it constitute the rotational angle detector 11 for ≤1 turn, and its output and the output of the counter circuit 7 are inputted to the output circuit 13 in parallel. Thus, the consumption of the battery is reduced and the economical, small-sized device is constituted.