To provide a sun sensor which is completely free from the effect of environmental light such as reflected light from a satellite construction body by enabling detecting of the angle of the sun accurately even in the environment where the intensity of the sun light is weak as in an outer planet.

An endless belt 20 having two incident slits 22 and 24 is arranged and a time length is measured to a timing at which the sunlight passes through the two incident slits 22 and 24 from the timing at which a pulse indicating the reference position of the incident slits has been outputted to calculate the angle of the sun. Since the angle of the sun is computed as a function of time, it does not depend on the intensity of the sunlight. When the frequency of detecting the sun during the one sampling cycle is more than once, the data becomes invalid. Thus, the environmental light is shielded to eliminate a shielding plate thereby miniaturizing the sensor.