To provide a rotational amount measuring instrument which is improved in environmental resistance and can detect and measure the rotational amount, such as the rotational position, number of revolutions, etc., of a rotating body with high accuracy.

A rotational amount measuring instrument A which measures the rotational amount, such as the number of revolutions, rotational position, etc., of the rotating shaft of a motor M, such as the induction motor, servo motor, etc., is constituted in such a way that the magnetic flux passing through a magneto-resistance element block is made to change when the rotating shaft is rotated and magneto-resistance elements 13A, 13B, 13C, and 13D convert the change of the magnetic flux which occurs between the gear 11 and magnets 14 faced oppositely to the teeth of the gear 11 when the rotating shaft is rotated into two voltage changes of different phases, and then, the rotational amount, such as the rotational position, number of revolutions, etc., of the rotating shaft are calculated from the voltage changes.