Smoke sensor including a current to voltage circuit having a low frequency correction means to produce a correction current

A current-voltage converting circuit (2) is provided with a first feedback circuit (5) and a correcting transistor (Q1). The first feedback circuit (5) outputs, of an output voltage (V10), a voltage according to a magnitude of a low-frequency component that is not greater than or equal to a predefined first cut-off frequency. The correcting transistor (Q1) extracts a correction current (I21) according to a magnitude of an output of the first feedback circuit (5) from a sensor current (I10). The first feedback circuit (5) has a first integrating circuit (9) and a sample-and-hold circuit (10). The first integrating circuit (9) integrates the output voltage (V10) of a conversion section (3). The sample-and-hold circuit (10) samples and holds an output of the first integrating circuit (9) during a sensing period at which a pulsed detection signal is inputted. Means for preventing an incidence of ambient light onto a light-receiving section can be simplified or omitted as a result.