Patent ID: 8283915

Claim:
A gear mechanism comprising: a gear that transmits rotation to an output shaft; a sensor magnet device that is placed at one axial end side of the gear, wherein the sensor magnet device includes at least one primary magnet portion that includes a magnetic material and is magnetized to have a predetermined magnetizing pattern, wherein the at least one primary magnet portion is placed along a predetermined first imaginary circle, a center of which coincides with a rotational axis of the gear, at least one secondary magnet portion that includes a magnetic material and is magnetized to have a predetermined magnetizing pattern, which is different from the predetermined magnetizing pattern of the at least one primary magnet portion in a circumferential direction, wherein the at least one secondary magnet portion is placed along a predetermined second imaginary circle, which is coaxial with the first imaginary circle along a common axis that is coaxial with the rotational axis of the gear and has a diameter that is smaller or larger than that of the first imaginary circle, and the at least one secondary magnet portion and the at least one primary magnet portion are arranged at different radial distances from the common axis, and at least one connecting portion that is formed integrally with and connects between the at least one primary magnet portion and the at least one secondary magnet portion, wherein the sensor magnet device is formed such that an external magnet sensor outputs a signal corresponding to a rotational state of the output shaft based upon a sensing by the external magnet sensor of at least one of the at least one primary magnet portion and the at least one secondary magnet portion; and a fixing means for fixing the sensor magnet device to the gear at the at least one connecting portion.