In case of developing an image output apparatus including a controller to generate image data and an engine to form an image on a medium based on the image data, in order to verify the image data from the controller in real time, halftone image data before conversion into a PWM signal is converted into a serial signal and output instead of the PWM signal for output to the engine unit if a controller operation verification apparatus is used instead of the engine unit. Thus, multivalued halftone data not including an error due to a conversion process can be sent to the controller operation verification apparatus without adding signal line and driver circuit dedicated for sending the multivalued halftone data, whereby it is possible to improve development efficiency of the image output apparatus by accurate verification of the output image data.