To provide a reduction gear unit with a rotational position sensor having high moment rigidity.

A geared motor 1 is comprised of a motor unit 2 and a reduction gear unit 4. The reduction gear unit 4 is provided with a harmonic drive 7 as a reduction gear, an output shaft 8, that is, a reduction gear output element and integrally formed with a flexible external gear 72 with a cup-like shape, and an absolute sensor 20 for detecting the rotational position of the output shaft 8. The absolute sensor 20 is disposed in the part of the output shaft 8 supported in a both-end supported state by the flexible external gear 72 and an output side bearing 12. The absolute sensor 20 is disposed without lowering the moment rigidity of the output shaft 8.

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