A restriction rotating body is provided at a carrier plate, on the opposite side of a sun gear to a planet gear. When an internal-toothed gear is displaced together with a sensor gear in the rotation radial direction thereof, the internal-toothed gear presses a shaft portion of the restriction rotating body, and further, a gear portion of the restriction rotating body presses the sun gear. Due thereto, the amount of relative displacement of the internal-toothed gear with respect to the sun gear can accordingly be reduced. Moreover, since the restriction rotating body does not mesh with the internal-toothed gear, the restriction rotating body can be easily assembled after assembly of the planet gear. The planet gear can accordingly be easily assembled prior to assembly of the restriction rotating body.