Image forming apparatus and method of detecting error of printing process

In a copying machine having an image memory that stores image data, a DRAM control IC that repetitively reads out image data from the image memory, and an image forming section that forms the images for the specified number of copies, a write processing section having a number of pixels counter that counts the number of pixels included in the image data for the predetermined number of pages at the time of reading, a first memory that stores the count value for the n th copy, a second memory that stores the count value for the (n + 1) th copy, and an image control CPU that compares the count value stored in the first memory with the count value stored in the second memory and thereby judges the occurrence of errors in the print process if the two counts are not equal to each other and carries out error processing.