A controller works with a measure of temperature of ink at a temperature sensor within a first temperature range to have an inkjet head propel out droplets of ink at a first discharge speed for formation of images, and with a measure of temperature of ink at the temperature sensor within a second temperature range lower than the first temperature range to have the inkjet head propel out droplets of ink at a second discharge speed greater than the first discharge speed for formation of images, and the controller determines that end regions of a frame of images to be formed in regions at transfer-longitudinal ends of a sheet to be transferred in a sheet transfer system have a print ratio within a prescribed range, to execute formation of images.