Provided is a thermal printer in which positioning precision between a platen roller and a thermal head is enhanced, thereby being capable of positively arranging heating elements within a heating element allowable range. A positioning mechanism (38) includes: a through-hole (40) which allows a head support member (23) to be movable with respect to a detachable frame (21); a regulating surface (44a) formed in the head support member (23), an elastic member (42) for urging the head support member (23) so that the regulating surface (44a) is abutted to a bearing (18) of the platen roller (6); and a guide portion (44b), which is formed in the head support member (23), for guiding the head support member (23) by moving the head support member along the through-hole (40) in association with a mounting operation of a cover unit (4) while sliding with the bearing (18) so that the regulating surface (44a) abuts the bearing (18), and the regulating surface (44a) is formed in a flat surface, and a normal line direction of the regulating surface (44a) coincides with a direction perpendicular to an axial direction of the platen roller (6) within a plane which is parallel with a head surface 19a of a thermal head (19).