A signal processing circuit provided with an outside-light component reducing circuit capable of reducing an outside light component in the frequency separation method is provided. This is a signal detecting circuit which irradiates a pulse light from an LED to a windshield of a vehicle, receives a reflected light by a PD, processes the pulse signal from the PD and inputs it to a processing unit in order to automatically control a wiper for eliminating raindrops on the windshield of the vehicle. This is provided with a current--voltage converter circuit for converting the pulse signal from the LED to a voltage signal, an outside-light component reducing circuit provided in parallel with the current--voltage converter circuit for reducing the outside light component included in the output signal of the current--voltage converter circuit and a band-pass filter circuit/amplifier circuit for reducing the noise in the output signal of the current--voltage converter circuit and amplifying the output signal.