To provide an image data processing circuit with high productivity and a color image forming apparatus using the same by enabling sharing of the image data processing circuit between a four-rotation machine and a quadruple tandem machine, and reduction of registers for four colors arranged for the four-rotation machine to a single register for one color. There is provided an image forming apparatus in which a first image processing unit out of first and second image processing units provided in an image data processing circuit executes a first image processing for an image read by a scanner unit, stores an image signal in a page memory, the second image processing unit reads the image signal from the page memory according to a synchronous signal generated in an engine unit to execute a second image processing, and the engine unit forms the image on the basis of the results therefrom.