PURPOSE: To obtain a sharp image, by a method wherein a thermal head is fixed to an apparatus main body frame and a back roll is energized by a spring to be brought into contact with said thermal head to prevent color shift.

CONSTITUTION: A thermal head 41 and a back roll 42 are mounted to a frame body 43 to form a unit which is, in turn, subjected to positional determination to be provided to an apparatus main body frame 44. The thermal head 41 are fixed in a suspended state between both side plates 45, 46 of the frame body 43 while the back roll 42 is mounted in such a state that the shaft 55 thereof inlaid with and mounted to bearing notches 51, 52 to be energized by springs 56, 57. On the other hand, positioning pins 60, 61 directed to a same direction are provided to the side plates 58, 59 of the apparatus main body frame 44 in a protruded state. Because the thermal head 41 is fixed to the apparatus main body frame 44, positional relation between a plurality of thermal heads is not shifted and, therefore, multicolor registration is certainly coincided.