An image forming device includes: an endless transport belt in which a plurality of through holes are formed, the transport belt circulating to carry sheets; a recording head with a plurality of nozzles through which ink droplets are discharged, the nozzles being arranged in a width direction of the transport belt. The image forming device performs preliminary discharge of ink droplets in which the ink droplets discharged through the nozzles pass through the through holes. The image forming device further includes: a sensor that detects an element to be detected formed on the transport belt when the transport belt circulates; and a preliminary discharge control unit that controls timings of discharge of ink droplets through the nozzles in the preliminary discharge based on a plurality of results of detecting the elements to be detected given from the sensor.