This paper presents the structure and application of a sun tracking photo-sensor, which controls the direction of a reflected solar radiation to be constant. The sensor has a very simple structure, in which two photo-cells are placed side by side in the bottom of a box with an aperture. When the reflected solar radiation passes through the aperture, the photo-cells are fractionally illuminated, and a electric current, which is proportional to the illuminated area, is generated on each photo-cell. The direction of the reflected solar radiation is kept to be constant by controlling the angle of the reflection mirror to make the two currents equal. The performance of the photo-sensor was tested with a mirror-telescope system and the sun tracking error was estimated to be less than 0.6 mrad on sunny day. To ascertain the applicability of the photo-sensor, a sensor-controlled heliostat, in which no computer-control system and rotary encoders were installed, was manufactured. The performance of the heliostat was evaluated by observing the radiation on the target screen which was placed 70 m south side of the heliostat, and the angular error within 2 mrad was achieved on sunny days, while the tracking error increased in various ways in cloudy weather. This photo-sensor is applicable for small solar furnace, sun lighting system, and sun tracking photovoltaic system.