An alarm system for serving notice of abnormalities such as fire, burglary or other malfunctions has extinction-type separated smoke detectors which detect smoke by exploiting the phenomenon that the intensity of a light beam emitted by a light transmitter toward a light receiver remote from the light transmitter undergoes attenuation due to the occurrence of smoke. The alarm system utilizes the light beam as a carrier for transmitting a signal indicative of an abnormality, thereby reducing the number of line conductors required for signal transmission in this kind of alarm system while signal transmission is rendered less susceptible to the influence of electromagnetic interference. The light transmitter is provided with a signal input circuit composed of an encoder and a serial-to-parallel conversion circuit for receiving the signal to be transmitted and a modulating circuit for modulating a current for driving the light transmitter with the output signal of the signal input circuit. The light receiver includes a demodulator circuit for detecting the modulated signal from the output signal of a photo-electric element and a signal output circuit composed of a serial-to-parallel conversion circuit and a decoder for restoring the signal for transmission from the output of the modulator circuit. When a plurality of these smoke detectors are connected in series, the abnormality alarm signal is transmitted through the light beams in each of the detectors to a receiver unit.