A pen printer for performing drawing with a detachably held pen comprises a pen holder operation mechanism for moving a pen holder detachably holding the pen between a pen attaching position, where the pen is attached to or detached from the pen holder, and a home position, where drawing is started or ended, and for moving the pen holder from the home position for drawing a desired sign or figure according to a pattern corresponding to the desired sign or figure and causing the pen holder to return to the home position, a support member for supporting the pen holder operation mechanism to be movable in all directions, a pen seat, mounted on the support member, for receiving a tip of the pen held by the pen holder, pen attaching position reference point display marks, mounted on the pen seat, for displaying a pen attaching position reference point for the pen tip of the pen held by the pen holder, and pen holder operation mechanism moving screws for moving the pen holder operation mechanism in all directions so as to align the pen tip of the pen hold by the pen holder in the pen attaching position with the pen attaching position reference position.