A thermal printer module includes: a housing; a cover; a platen roller provided to the cover; a support frame having a groove configured to receive a shaft of the platen roller; a lock arm configured to hold down, by a platen roller engagement portion, the shaft of the platen roller inserted into the groove; a biasing member configured to apply a biasing force to the lock arm; and a printing head. Wherein, when viewed in a direction extending along an axis of the shaft, the platen roller engagement portion and a tangential line of a track obtained when the axis of the shaft moves along with an opening operation of the cover form an intersection angleθAB in a closing direction of the lock arm, and the intersection angle θAB satisfies a relation of 90°≤θAB≤110°. Wherein the platen roller engagement portion and a tangential line of a track obtained when a contact point between the shaft and the platen roller engagement portion pivots in an unlocking direction of the lock arm form an intersection angle θBC in the closing direction of the lock arm, and the intersection angle θBC satisfies a relation of 0°≤θBC≤10°.