A rain sensor has a frost sensing function, and is attached to a windshield glass of a vehicle. The frost sensor includes a casing provided with a frost sensing region formed by recessing a part of the casing to contact indoor air; a light emitting unit that emits light such that the light is incident on the windshield glass; a light receiving unit that receives the light emitted from the light emitting unit and reflected from the windshield glass and the frost sensing region; and a control unit that outputs the light received by the light receiving unit as a control signal and activates a wiper and an air conditioner of the vehicle.