A rotational angle detector provided with a circular rotary plate integrally rotatable with a rotor of a motor and including a circumferential surface having marks, which are spaced apart by different angular intervals in a circumferential direction. A rotational angle detection unit is arranged along the circumferential direction of the rotary plate to detect a rotational angle of each mark, rotated together with the rotary plate, in a predetermined detection zone. The rotational angle detector determines, when adjacent marks are located in the detection zone, angular intervals between the adjacent marks based on the rotational angle of each mark, specifies a mark that is located in the detection zone based on the determined angular intervals, and detects the rotational angle of the rotor based on the rotational angle of the specified mark rotating in the detection zone.