In a smoke sensor, a first light-receiving element which receives light from a light-emitting element and is disposed in a position where the quantity of received light changes according to a density of smoke and a second light-receiving element which monitors the quantity of light of the light-emitting element are arranged symmetrically with respect to the light-emitting element. Furthermore, a signal from the first light-receiving element and a signal from the second light-receiving element are amplified by an identical amplifier circuit. In a microcomputer, the density of smoke is computed based on a difference between an output obtained by amplifying an output of the first light-receiving element with the amplifier circuit and an output obtained by amplifying an output of the second light-receiving element with the amplifier circuit.