An image forming apparatus includes an engine for forming an image corresponding to received video data on a sheet of paper, a main frame buffer memory, an auxiliary frame buffer memory for outputting stored video data along a data transmission path to the engine according to a video clock sync signal, and a first signal when a lower level correspondingly set to at least some amount of the stored video data is reached and a second signal when the stored data reaches a set upper level. This image forming apparatus further comprises a central processing unit for converting print data received externally to video data suitable for driving of the engine and storing the converted video data in the main frame buffer memory, and a direct memory access controller for controlling the main frame buffer memory so that video data stored in the main frame buffer memory is transmitted to the auxiliary frame buffer memory until the second signal is output when a print start signal is input, at an initial stage, and the video data stored in the main frame buffer memory is transmitted to the auxiliary frame buffer memory as much as a set data transmission amount whenever the first signal is input, at a subsequent stage after the initial stage. Also provided is a video sync signal controller for controlling operation and interruption of the direct memory access controller according to a video data output control signal output from the engine and outputting the video clock sync signal. Thus, as the video data can be continuously output to the engine in synchronization with the video clock signal until the print stop signal is generated, speed of print can be increased. Further, a large capacity of storing video data for a memory for video data transmission is not needed.