An alarm unit of the invention is provided with: a light emitting section; a battery power supply; a voltage booster circuit that boosts a voltage from this battery power supply to thereby generate a boosted voltage; a light emission control section that controls the voltage booster circuit to supply, with the timing at which the boosted voltage is obtained, the boosted voltage to the light emitting section, thereby intermittently driving light emission; a light receiving section that receives light from the light emitting section having been diffused by smoke; a conversion circuit that converts a received light signal from the light receiving section into received light data; a fire hazard detection section that detects a fire hazard based on the received light data from the conversion circuit; an alarm section that outputs an alarm based on a fire hazard detection signal from the fire hazard detection section; a reference voltage circuit that generates a reference voltage for the voltage booster circuit and the conversion circuit; and a clock circuit that outputs a clock signal for operating the voltage booster circuit, the light emission control section, and the conversion circuit. Furthermore, in this alarm unit, a packaged integrated circuit is provided with: a processor circuit that realizes functions of the light emission control section and the fire hazard detection section by executing a program; the voltage booster circuit; the conversion circuit; the reference voltage circuit; the clock circuit; and a control circuit of the respective circuit sections.