PROBLEM TO BE SOLVED: To make a thermal head locked at an optimum pressing force in a pressing state of a platen roller so as to print printing information highly precisely by providing a first cassette for rotatably supporting a heat sink to be attached to the thermal head and a second cassette for supporting the platen roller. SOLUTION: A first cassette (main body) 17 which supports a heat sink (heat radiating plate) 3 is rotatably provided. In the main body 17, a pressing spring 11 for pressing a thermal head 1 itself to a platen roller 2 through the heat sink 3 is provided. Also, there is provided a rear case 5 which supports the platen roller 2 rotatably and has by itself a rotation supporting point 6. For fitting the rear case 5 in the main body 17, a lock pin 9 is fixed to the rear case 5. On the other hand, a lock claw 8 (fork) is provided at a part of the heat sink 3. The heat sink 3 is inclined by a lock release lever, so that the fitting of the lock pin 9 and the lock claw 8 is released so as to rotate a second cassette 18.