A rotation angle detecting device includes a rotating body rotating according to rotation of a vehicle pedal; a magnet attached to the rotating body; a magnetism detection element disposed facing the magnet at a predetermined interval from the magnet; and a first control circuit connected to the magnetism detection element. The center of the magnet is placed at the rotation center of the rotating body. The magnetism detection element detects magnetism of the magnet to output a detection signal. The first control circuit outputs an angle signal corresponding to a rotation angle of the rotating body according to the detection signal, and outputs a switching signal according to the rotation angle of the rotating body. The rotation angle detecting device reliably detects a rotation angle and turns a stop lamp on and off for example with a simple structure.