A thermal printer enables both duplex printing and simplex printing. A double-sided thermal printer 1 has a first arm 31 and a second arm 32 that opens and closes in conjunction with the first arm 31. A first platen roller 21 is mounted on the first arm 31, a second thermal print head 12 is mounted on the second arm 32, and a first thermal print head 11 and second platen roller 22 are disposed on the printer frame 30 side. When the operating cover 3 closes, the second thermal print head 12 on the second arm 32 is pressed against the second platen roller 22, the first platen roller 21 on the first arm 31 is pressed to the first thermal print head 11, and the double-sided thermal paper 6 can be set with no slack. For simplex printing, the paper transportation load can be reduced by pressing only the first platen roller 21 on the first arm to the first thermal print head 11.