An image forming apparatus includes an image forming part for forming an image described by a video signal on a recording sheet by carrying out an image forming operation which includes scans in main and sub scan directions, a storage part for storing test pattern data which describes at least one test pattern to be formed on the recording sheet, a generator for generating a generation interval of the video signal in the main and sub scan directions responsive to the test pattern data stored in the storage part, and a controller for controlling the image forming operation of the image forming part by supplying to the image forming part the video signal which describes an arbitrary test pattern and is generated based on the test pattern data read from the storage part with the generation interval generated by the generator.