A thermal printer includes a support frame and a head frame supported by the support frame. A line thermal head is attached to the head frame. A platen unit is mounted on the support frame and includes a platen roller opposing the thermal head and a pair of support members supporting both ends of the platen roller. The support members are arranged to be rotatable so that the platen roller is movable between a contact position where the roller contacts the thermal head and a separate position where the roller is set apart from the thermal head. A pressing mechanism is provided on the platen unit so as to press the platen roller onto the thermal head at a predetermined pressure.