To precisely detect a home position in a rotation direction of an intermediate transfer body without using an A/D port.

A color image forming device has a plurality of developing devices and the intermediate transfer body for multiple transfer of images developed on respective developing devices. The color image forming device is provided a detection means which is provided with a light emitting element and a light receiving element to detect a mark for locating the home position in the rotation direction of the intermediate transfer body, an input part which takes an analog signal from the light receiving element of the detection means as an input to discriminate the level, a driving part which controls the light emission quantity of the light emitting element, and a control means which operates the driving part. The control means examines values of the signal from the input part, which has the level discriminated and includes a digital value representative of the mark for locating the home position and digital values representative of the other parts, for a period of at least one rotation at the time of rotatively driving the intermediate transfer body and controls operation of the driving part so that the analog signal from the light receiving element may have a preliminarily set amplitude or level.

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