To provide a simple and compact automatic exposure controller.

A switching plate 31 where a large diaphragm aperture 47 and a small diaphragm aperture 46 are formed is turnably provided between a 1st position where the aperture 46 is inserted in a photographing optical path and a 2nd position where the aperture 46 is retreated from the photographing optical path and the aperture 47 is inserted therein. The plate 31 is energized to the 2nd position by a spring 53. When a shutter blade 34 is at a closed position, it abuts on the plate 31 and holds it at the 1st position. Then, an iron plate 49 is provided at one end of the plate 31. When a solar battery 16 receives fixed quantity of light, an electromagnet 39 is actuated and attracts the plate 49, whereby the plate 31 is held at the 1st position even when the blade 34 is opened.