A printer unit comprising: a platen roller including a driven gear; a drive source configured to rotate the platen roller about a predetermined axis; a frame, which is configured to rotatably support the platen roller and to which the drive source is assembled; a reduction gear (65, 66) configured to transmit a driving force of the drive source to the driven gear in a decelerated manner; a thermal head to be held in press contact with an outer peripheral surface of the platen roller; a gearbox portion, which is formed on the frame and to which the driven gear and the reduction gear are assembled, the gearbox portion having an opening for assembling the reduction gear; and a gear cover configured to close at least a part of the opening of the gearbox portion, the gearbox portion having a recessed portion recessed in a direction intersecting an axial direction of the predetermined axis, the gear cover including: a hook portion (24, 25) to be engaged with the gearbox portion; and a projecting portion (26, 27) to be fitted to the recessed portion of the gearbox portion, wherein the gear cover is formed to be elongated in a predetermined direction when viewed in the axial direction, the hook portion (24, 25) is formed on a side opposite to the projecting portion across an intermediate portion of the gear cover in the predetermined direction when viewed in the axial direction, characterized in that the projecting portion (26, 27) is formed on the driven gear side with respect to the intermediate portion of the gear cover in the predetermined direction when viewed in the axial direction, and the recessed portion is recessed in substantially the predetermined direction, whereas the projecting portion (26, 27) protrudes from the gear cover in substantially a fore-and-aft direction that is perpendicular to the predetermined direction. a platen roller including a driven gear; a drive source configured to rotate the platen roller about a predetermined axis; a frame, which is configured to rotatably support the platen roller and to which the drive source is assembled; a reduction gear (65, 66) configured to transmit a driving force of the drive source to the driven gear in a decelerated manner; a thermal head to be held in press contact with an outer peripheral surface of the platen roller; a gearbox portion, which is formed on the frame and to which the driven gear and the reduction gear are assembled, the gearbox portion having an opening for assembling the reduction gear; and a gear cover configured to close at least a part of the opening of the gearbox portion, the gearbox portion having a recessed portion recessed in a direction intersecting an axial direction of the predetermined axis, the gear cover including: a hook portion (24, 25) to be engaged with the gearbox portion; and a projecting portion (26, 27) to be fitted to the recessed portion of the gearbox portion, wherein a hook portion (24, 25) to be engaged with the gearbox portion; and a projecting portion (26, 27) to be fitted to the recessed portion of the gearbox portion, wherein the gear cover is formed to be elongated in a predetermined direction when viewed in the axial direction, the hook portion (24, 25) is formed on a side opposite to the projecting portion across an intermediate portion of the gear cover in the predetermined direction when viewed in the axial direction, characterized in that the projecting portion (26, 27) is formed on the driven gear side with respect to the intermediate portion of the gear cover in the predetermined direction when viewed in the axial direction, and the recessed portion is recessed in substantially the predetermined direction, whereas the projecting portion (26, 27) protrudes from the gear cover in substantially a fore-and-aft direction that is perpendicular to the predetermined direction.