A rider separation detecting device for precisely and quickly detecting the separation of a rider from a rider's seat. The rider separation detecting device includes a cable connected at one end thereof to one of a garment worn by the rider and a vehicle body, and winding mechanism having a drum connected to the other end of the cable and rotationally biased in a direction of winding of the cable. The winding mechanism is provided on the other of the garment and the vehicle body. Also included are a rotational angle detecting mechanism for detecting a rotational angle of the winding mechanism, and separation determining mechanism for determining the separation of the rider from the rider's seat according to a detection value from the rotational angle detecting mechanism. The device ensures proper detection and a good external appearance without hindering a normal action of the rider on the rider's seat.