In a thermal printer for feeding a direct thermal paper or a thermal transfer ribbon and a printing paper between a thermal head and a platen roller while such paper is pressed and held by pressing force of a pressure application means, and printing on such paper by heating a heating portion of the thermal head, the positioning of the thermal head relative to the platen roller in the paper feeding direction can be easily performed by a simple mechanism without using an exclusive assembling instrument. To achieve this object, the thermal head and the platen roller are attached to the same member or members which are integrated with each other and there is provided a linear moving mechanism for moving the thermal head and the platen roller toward or away from each other while one of the thermal head and the platen roller is held by the pressure application means so as to be movable linearly in the pressure application direction, whereby the thermal head is brought into contact with the platen roller or vice versa by merely linearly moving the thermal head or the platen roller.