To provide an image forming apparatus which saves a useless electric power, ensures a life of a printer, and is convenient for a user, without being influenced by an environment for use and a setting condition by making it possible to perform ON/OFF of an electric source of the printer by reacting to an indoor illumination, and to provide a method for saving electric power by the apparatus.

The image forming apparatus is equipped with an image forming part for forming an image on a recording medium, an electric power circuit part for feeding an electric power to the image forming part, and a light volume detecting part for detecting an external light volume and outputting a light receiving signal in accordance with the light volume to the electric power circuit. The electric power circuit part is characterized in that feeding, shutting or waiting for the electric power to the image forming part is performed based on the light receiving signal outputted by the light volume detecting part.