Abstract: A photoelectric intrusion detector for detecting the interruption by an intruder of a monitoring beam of optical radiation such as an infrared radiation includes circuitry for preventing the generation of a false alarm when the optical beam is attenuated during its propagation through space due to fog or the like. This false alarm preventing circuitry is designed so that when the attenuation of a beam of optical radiation during its propagation through space is increased due to the occurrence of fog in cloudy weather, the occurrence of a false alarm due to the attenuation of the pulsed light in cloudy weather is prevented by decreasing a comparator reference value to follow the decrease in the level of a light receiving signal, correcting the gain of AGC amplification to maintain constant the level of a received light signal or correcting the level of a received light signal which follows a received light initial value.