A movable-body-speed-detecting apparatus includes a movable body capable of moving linearly and/or circularly, a plate fixed to the movable body, the plate having a predetermined length in a direction in which the movable body moves, and light emission/reception means arranged such that light output from a light-emitting element enters a light-receiving element. The plate has a window section that passes between the light-emitting element and the light-receiving element in accordance with movement of the movable body and that blocks or allows passage of light to the light-receiving element. The apparatus further comprises speed calculation means for calculating the speed of the movable body by converting the amount of light received by the light-receiving element into a signal and measuring the signal.