A printer comprises image signal generating element for repeatedly generating one line of image signal, compare element having a plurality of reference levels and comparing the image signal with selected one of the reference levels, thermal head element having a line of heating elements for printing one line of image signal, current supply element for supplying currents to the heating elements in accordance with the compare results, setting element for setting a continuous energization period for supplying the current to the heating elements from the supply element, and control element for controlling the supply element to provide a first period during which the energization of the heating elements in a first group is inhibited and a second period during which the energization of the heating elements in a second group are inhibited.