A state detecting device comprising: a light-emitting element for outputting pulse light; and a light-receiving element for receiving pulse light emitted from the light-emitting element, wherein a state of a detection target, in particular a rain drop, existing between the light-emitting element and the light-receiving element is detected based upon an electrical light-receiving signal outputted from the light-receiving element, in which outside optical signal elements (Fg) are superimposed on pulse optical signal elements (Fp), the pulse optical signal elements (Fp) being a component of the electrical light-receiving signal resulting from the influence of the state of the detection target on the electrical light-receiving signal, and the outside optical signal elements (Fg) being a component of the electrical light-receiving signal resulting from the temporal alteration of outside influences other than the state of the detection target on the electrical light-receiving signal, the outside optical signal elements (Fg) being in a lower frequency region than the pulse optical signal elements (Fp), the state detecting device comprising: a current to voltage conversion circuit for converting the electrical light-receiving signal from current to voltage; a high pass filter circuit (20, 20A; 40, 40A) for extracting the pulse optical signal elements (Fp) from the electrical light-receiving signal, the high pass filter circuit (20, 20A; 40, 40A) generating undershoots (ΔVp -under) by the passing of the electrical light-receiving signal through the high pass filter circuit; a peak hold circuit for peak-holding the electrical light-receiving signal passing through the current to voltage conversion circuit and the high pass filter circuit (40, 40A); a non-reverse amplifying circuit for amplifying the output of the peak hold circuit; state determining means for determining the state of the detection target based upon a peak value (ΔVsig) of the electrical light-receiving signal amplified in the non-reverse amplifying circuit; and offset means (31, 41) for offsetting the electrical light-receiving signal by adding an offset voltage such that the minimum voltage of an undershoot (ΔVp -under) generated by the passing of the electrical light-receiving signal through the high pass filter circuit (20, 20A; 40, 40A) is equal to or greater than a minimum input voltage which can be inputted to the peak hold circuit, wherein the non-reverse amplifying circuit includes an offset calibrating unit to offset the electrical light-receiving signal in an opposite direction for absorbing the offset voltage of the electrical light-receiving signal, thus amplifying the output of the peak hold circuit and outputting an amplifying peak value (ΔVsig). a light-emitting element for outputting pulse light; and a light-receiving element for receiving pulse light emitted from the light-emitting element, wherein a state of a detection target, in particular a rain drop, existing between the light-emitting element and the light-receiving element is detected based upon an electrical light-receiving signal outputted from the light-receiving element, in which outside optical signal elements (Fg) are superimposed on pulse optical signal elements (Fp), the pulse optical signal elements (Fp) being a component of the electrical light-receiving signal resulting from the influence of the state of the detection target on the electrical light-receiving signal, and the outside optical signal elements (Fg) being a component of the electrical light-receiving signal resulting from the temporal alteration of outside influences other than the state of the detection target on the electrical light-receiving signal, the outside optical signal elements (Fg) being in a lower frequency region than the pulse optical signal elements (Fp), a current to voltage conversion circuit for converting the electrical light-receiving signal from current to voltage; a high pass filter circuit (20, 20A; 40, 40A) for extracting the pulse optical signal elements (Fp) from the electrical light-receiving signal, the high pass filter circuit (20, 20A; 40, 40A) generating undershoots (ΔVp -under) by the passing of the electrical light-receiving signal through the high pass filter circuit; a peak hold circuit for peak-holding the electrical light-receiving signal passing through the current to voltage conversion circuit and the high pass filter circuit (40, 40A); a non-reverse amplifying circuit for amplifying the output of the peak hold circuit; state determining means for determining the state of the detection target based upon a peak value (ΔVsig) of the electrical light-receiving signal amplified in the non-reverse amplifying circuit; and offset means (31, 41) for offsetting the electrical light-receiving signal by adding an offset voltage such that the minimum voltage of an undershoot (ΔVp -under) generated by the passing of the electrical light-receiving signal through the high pass filter circuit (20, 20A; 40, 40A) is equal to or greater than a minimum input voltage which can be inputted to the peak hold circuit,