In an image forming device, when a priority of outputting input data determined by a priority determining unit is higher than a priority of in-process data during image expansion performed by one of PDL interpreter units, a PDL control unit determines whether the one of the PDL interpreter units supports an interruption process, and an output sequence changing unit changes an output sequence stored in an output sequence storing unit such that, when the one of the PDL interpreter units supports the interruption process, another of the PDL interpreter units performs image expansion of the input data earlier than the image expansion of the in-process data, and when the one of the PDL interpreter units does not support the interruption process, another of the PDL interpreter units performs the image expansion of the input data later than the image expansion of the in-process data.