PURPOSE: To reduce the power consumption of the detector by a method wherein a voltage is applied to a magnetism-reactive element for rotation-direction detection use only when the state of a magnetism-reactive element for rotation detection use is changed.

CONSTITUTION: This detector is provided with a shaft 1 to which a permanent magnet 2 is attached, with a magnetism-reactive element 11, for rotation detection use, which is arranged and installed so as to be close to the permanent magnet 2 and with a magnetism-reactive element 21, for rotation-direction detection use, which is arranged and installed in a position whose phase is different from that of the magnetism-reactive element for rotation detection use. The detector detects the rotation of the shaft 1 from a change in the state of the magnetism-reactive element 11, and it detects a rotation direction from a change in the state of the magnetism-reactive element 21 A sampling voltage is applied to the magnetism-reactive element 11 from a power- supply circuit 30, the state of the magnetism-reactive element is monitored, a voltage is applied to the magnetism-reactive element 21 only when the state of the magnetism- reactive element for rotation detection use is changed. When the output of a latch circuit 90 is at a high level (i), it is judged that the shaft is turned forward. When the output is at a low level (j), it is judged that the shaft is turned in the reverse direction.