A rotation detecting unit includes a light emitting device, a light receiving device, an output device, a rotatable plate, and a discriminating device. The rotatable plate includes a first portion and a second portion which are alternately provided along a circumferential direction, a third portion provided between the first portion and the second portion, and a fourth portion provided between the second portion and the first portion. When a region of the first portion is irradiated with light emitted from the light emitting device and received by the light receiving device, received light quantity of the light receiving device is a maximum. When a region of the second portion is irradiated with the light, the received light quantity is a minimum. When the third or fourth portion is irradiated with the light, the received light quantity is larger than the minimum and is smaller than the maximum.