A thermal printer avoids or reduces difficult to correct problems caused by recording paper that has passed the printing position leaving the paper transportation path. A printing mechanism has a head mounting plate that carries a line thermal head, a front guide plate disposed downstream from the head mounting plate, a platen roller that defines the printing position A, and a back guide member disposed downstream from the platen roller. The head mounting plate and front guide plate have interlocking parts that interlock with each other widthwise to the paper transportation path. A paper detector that detects entry of the recording paper 45 is disposed to a gap E between the back guide member and platen roller. When the paper detector detects the recording paper 45, a rotation control unit stops the platen roller.