To substantially lower conventional restrictions on a minimum rotational frequency by detecting rotational frequencies even in a low-speed range in which rotational frequencies can not have been detected by a conventional shape, reduce the number of coil windings more than before by implementing the present invention, reduce the outside dimension of a magnet, and improve the function of and reduce the size of a power tool to which an electric motor control circuit is mounted.

A magnetic flux path is formed in such a way as to pass through the north pole or south pole and the iron core 4 of a permanent magnet 6 with the rotation of a rotator 5a of a motor 5, and the magnetic flux path is made of an extension part 4a from the iron core 4.