A state detecting device comprising: a light-emitting element; and a light-receiving element for receiving pulse light emitted from the light-emitting element, wherein a state of a detection target existing between the light-emitting element and the light-receiving element is detected based upon a received light signal that is output from the light-receiving element, the state detecting device comprising: a current to voltage conversion circuit for converting the received light signal from current to voltage; a peak hold circuit for peak-holding a peak value of the received light signal passing through the current to voltage conversion circuit; a range switching amplifying circuit (50, 50A) including an amplifier for switching the peak value output ef by the peak hold circuit to at least two amplification ranges for amplification ((Vout); (Vout); an enlarged range signal generating unit configured to generate as an enlarged range peak value the combined signal (Vf) which is the sum of a predetermined offset voltage (Vos) and the peak value (Vout) of the one received light signal amplified by the range switching amplifying circuit (50, 50A) when the peak value (Vout) of the one received light sig- nal amplified by the range switching amplifying circuit (50, 50A) is not saturated from a possible output range (Vc) of the amplifier, and the combined signal (Vf) which is the peak value (Vout) of the other received light signal amplified by the range switching amplifying circuit (50, 50A) when the peak value of the one received light signal amplified by the range switching amplifying circuit (50, 50A) is saturated from a possible output range (Vc) of the amplifier; and a state determining unit configured to determine the state of the detection target based upon the enlarged range peak value (Vf). a light-emitting element; and a light-receiving element for receiving pulse light emitted from the light-emitting element, wherein a state of a detection target existing between the light-emitting element and the light-receiving element is detected based upon a received light signal that is output from the light-receiving element, the state detecting device comprising: a current to voltage conversion circuit for converting the received light signal from current to voltage; a peak hold circuit for peak-holding a peak value of the received light signal passing through the current to voltage conversion circuit; a range switching amplifying circuit (50, 50A) including an amplifier for switching the peak value output ef by the peak hold circuit to at least two amplification ranges for amplification ((Vout); (Vout); an enlarged range signal generating unit configured to generate as an enlarged range peak value the combined signal (Vf) which is the sum of a predetermined offset voltage (Vos) and the peak value (Vout) of the one received light signal amplified by the range switching amplifying circuit (50, 50A) when the peak value (Vout) of the one received light sig- nal amplified by the range switching amplifying circuit (50, 50A) is not saturated from a possible output range (Vc) of the amplifier, and the combined signal (Vf) which is the peak value (Vout) of the other received light signal amplified by the range switching amplifying circuit (50, 50A) when the peak value of the one received light signal amplified by the range switching amplifying circuit (50, 50A) is saturated from a possible output range (Vc) of the amplifier; and a current to voltage conversion circuit for converting the received light signal from current to voltage; a peak hold circuit for peak-holding a peak value of the received light signal passing through the current to voltage conversion circuit; a range switching amplifying circuit (50, 50A) including an amplifier for switching the peak value output ef by the peak hold circuit to at least two amplification ranges for amplification ((Vout); (Vout); an enlarged range signal generating unit configured to generate as an enlarged range peak value a state determining unit configured to determine the state of the detection target based upon the enlarged range peak value (Vf).