To surely detect an artificial light, and to surely discriminate day from night even when artificial illuminations, such as a street-lamp and the like, exist in the surrounding.

The subject device is composed of a solar cell 1, a charge-and- discharge preventive diode 2, a direct-current power source 3, a direct-current blocking capacitor 4 for taking out only an alternating-current component, rectifier diodes 5, 6, a smoothing capacitor 7, a time constant resistance 8, a threshold setting circuit 9, and an analog voltage comparator 10 as a comparison means. And, an alternative-current component detecting means is composed of the direct-current blocking capacitor 4, the rectifier diodes 5, 6 and the smoothing capacitor 7.