PURPOSE: To prevent the deterioratin of a picture, by applying preheating in response to a picture at the left end of an effective screen, in a printer printing a still picture signal by usng a heat-sensing head.

CONSTITUTION: A signal with a picture at the left end of the effective screen is supplied to registers 17a, 17b while one color print is finished and a platen is restored to the print start position, or during a waiting time while manual print start is indicated and the platen reaches the print start position, and a signal corresponding to the 1st approximate 1/10 period during the 32-gradation is applied to a sensing head 19 for attaining preheating. Thus, no preheating is performed for the part having thin density, allowing to avoid unnecessary printing.