To detect rotation speed of a wheel at high precision, protect a device from the external, and improve safety performance and external appearance.

A car speed detection device 40 for a power-assisted bicycle comprises a disc-shaped sensor plate 41 disposed between a hub 9 of a wheel and a brake device 10 provided on the outer side in an axial direction of it, and rotation-integrally fixed with the hub 9, and a pickup sensor 42 to read motion of parts 46 to be detected continuously formed on an outer circumference of the sensor plate 41. An outer diameter D1 of the sensor plate 41 is set to be smaller than an outer diameter D2 of a brake panel 26 (26a), and a circumferential wall 26c of the brake panel 26 is extended toward the sensor plate 41 to surround a circumference of the sensor plate 41. The pickup sensor 42 is fixed to a car width direction inner surface of the brake panel 26 (26b).