In an automatic exposure device for a copying machine in which the density of an original is detected by irradiating an original by light from a lamp and detecting of the reflecting light with an optical sensor, and in accordance with the detected signal, the exposure amount of the lamp is adjusted by means of an automatic voltage adjusting circuit, and which is equipped with a heater control unit for controlling the energization of a fixing heater for fixing a toner sensible image which has been transferred on paper and a processing circuit for controlling the automatic voltage controlling circuit and heater control section, the processing circuit controlling the heater control section and being operable to maintain the energizing state of the fixing heater in a definite state, while the operation of detecting the original's density occurs with a lamp lit. In this way, fluctuation in the voltage applied to the lamp is prevented from occurring at the time of detecting the density, and the intensity of light from the lamp is thereby kept constant.