An apparatus for delaying image data of a plurality of parallel bits representing tones includes a compressing part for decreasing the number of bits of image data to decrease the number of tones, a delay memory for delaying image data provided from the compressing part, an extending part for increasing the number of bits of the image data provided from the delay memory to the original number of bits, and a memory mounting amount detecting part for detecting the state of mounting in the delay memory and designating the number of bits to be delayed by the delay memory for the compressing part and the extending part according to the detected state of mounting.