A thermal printer module, comprising: a housing including a roll sheet receiving portion; a cover, which is mounted to the housing so as to be pivotable, and is configured to open and close the roll sheet receiving portion; a platen roller provided to the cover so as to be freely rotatable; a support frame, which is provided in the housing, and has a groove configured to receive a shaft of the platen roller to be inserted into the groove when the cover is closed; a lock arm, which is provided to the support frame so as to be pivotable, and is 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 in a direction of causing the lock arm to pivot so as to maintain holding of the shaft by the platen roller engagement portion; and a printing head provided at a position opposed to the platen roller having the shaft held in the groove, wherein, the lock arm has a through hole formed in a circular shape, and is supported by the support frame via an arm shaft coaxially fitted into the through hole, the lock arm further comprises at least two side plate portions configured to extend substantially parallel to side wall portions of the support frame, and a rear plate portion connecting the at least two side plate portions; the side plate portion comprises a plate-like portion, and the through hole being formed in a lower portion of the side plate; wherein, when the shaft of the platen roller is held by the lock arm, the shaft is in contact with he inner surface of the groove on the opposite side of the print head by the biasing force of the print 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 held in the groove moves along with an opening operation of the cover form an intersection angle θ AB in a closing direction of the lock arm, and the intersection angle θ AB satisfies a relation of 90°≤θ AB ≤110°, and wherein, when viewed in the direction extending along the axis of the shaft, the platen roller engagement portion and a tangential line of a track obtained when a contact point between the shaft held in the groove and the platen roller engagement portion pivots in an unlocking direction of the lock arm form an intersection angle θ BC in the closing direction of the lock arm, and the intersection angle θ BC satisfies a relation of 0°≤θ BC ≤10°. a housing including a roll sheet receiving portion; a cover, which is mounted to the housing so as to be pivotable, and is configured to open and close the roll sheet receiving portion; a platen roller provided to the cover so as to be freely rotatable; a support frame, which is provided in the housing, and has a groove configured to receive a shaft of the platen roller to be inserted into the groove when the cover is closed; a lock arm, which is provided to the support frame so as to be pivotable, and is 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 in a direction of causing the lock arm to pivot so as to maintain holding of the shaft by the platen roller engagement portion; and a printing head provided at a position opposed to the platen roller having the shaft held in the groove, wherein, the lock arm has a through hole formed in a circular shape, and is supported by the support frame via an arm shaft coaxially fitted into the through hole, the lock arm further comprises at least two side plate portions configured to extend substantially parallel to side wall portions of the support frame, and a rear plate portion connecting the at least two side plate portions; the side plate portion comprises a plate-like portion, and the through hole being formed in a lower portion of the side plate; wherein, when the shaft of the platen roller is held by the lock arm, the shaft is in contact with he inner surface of the groove on the opposite side of the print head by the biasing force of the print 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 held in the groove moves along with an opening operation of the cover form an intersection angle θ AB in a closing direction of the lock arm, and the intersection angle θ AB satisfies a relation of 90°≤θ AB ≤110°, and wherein, when viewed in the direction extending along the axis of the shaft, the platen roller engagement portion and a tangential line of a track obtained when a contact point between the shaft held in the groove and the platen roller engagement portion pivots in an unlocking direction of the lock arm form an intersection angle θ BC in the closing direction of the lock arm, and the intersection angle θ BC satisfies a relation of 0°≤θ BC ≤10°.