Image data representing one pixel line and supplied from a line memory are divided by a dividing circuit into three sets of image data which are stored in respective line memories. A switcher is controlled by a timing signal from a controller to successively supply the image data from the line memories to a thermal head driver. Based on the supplied image data, the thermal head driver enables a thermal head to record three lines on a thermosensitive recording medium. The thermosensitive recording medium now records thereon a high-quality image free of coarse image textures which has recorded image areas dispersed in one pixel interval in the direction in which the thermosensitive recording medium is fed.