It is prevented that a malfunction occurs because of a noise. A smoke detection section samples as a smoke detection signal a signal digitized after being output from a light reception section when a light emission section glows, and updates a previous smoke detection signal. A noise detection section samples as a noise detection signal a signal output from the light reception section when the light emission section is not driven to glow. When the noise detection signal exceeds a predetermined noise level, a noise de-influence unit disables updating of the previous smoke detection signal with the newly sampled smoke detection signal so as to process a noise.