A power conversion apparatus for a printer including a body casing, a cover frame, a printer head unit including a gear train for transferring print paper, a gear train for cutting printed paper, and a cutter for cutting printed paper, and a platen roller unit, the power conversion apparatus comprising: a power motor; and a power converter configured to selectively convert rotation power of the power motor into the gear train for transferring and the gear train for cutting, respectively, depending on a rotation direction of the power motor, the power converter comprising: a sun gear configured to receive power from the power motor; a planetary gear configured to receive a rotating force while being engaged with the sun gear and to selectively couple with the gear train for cutting and the gear train for transferring, respectively; a rotating link configured to constrain a position of the planetary gear; and a pressing means P configured to increase an adhesion between the sun gear and the rotating link; characterised in that the rotating link of the power converter is protrudedly formed with a rotating shaft part coupled with the sun gear and a gear shaft part coupled with the planetary gear wherein the pressing means P is inserted between the sun gear and the rotating shaft part of the rotating link; wherein the pressing means comprises: semi-arch shaped bushes formed in a pair to face each other and the semi-arch shaped bushes configured to be inserted between the rotating shaft part of the rotating link and the sun gear; a spring member configured to be mounted in a long-hole concave groove formed on an outer circumferential surface of the bushes; and a bush protrusion formed at one end of the bushes to be inserted into a protrusion insertion groove formed on the rotating link. a power motor; and a power converter configured to selectively convert rotation power of the power motor into the gear train for transferring and the gear train for cutting, respectively, depending on a rotation direction of the power motor, the power converter comprising: a sun gear configured to receive power from the power motor; a planetary gear configured to receive a rotating force while being engaged with the sun gear and to selectively couple with the gear train for cutting and the gear train for transferring, respectively; a rotating link configured to constrain a position of the planetary gear; and a pressing means P configured to increase an adhesion between the sun gear and the rotating link; a sun gear configured to receive power from the power motor; a planetary gear configured to receive a rotating force while being engaged with the sun gear and to selectively couple with the gear train for cutting and the gear train for transferring, respectively; a rotating link configured to constrain a position of the planetary gear; and a pressing means P configured to increase an adhesion between the sun gear and the rotating link; characterised in that the rotating link of the power converter is protrudedly formed with a rotating shaft part coupled with the sun gear and a gear shaft part coupled with the planetary gear wherein the pressing means P is inserted between the sun gear and the rotating shaft part of the rotating link; wherein the pressing means comprises: semi-arch shaped bushes formed in a pair to face each other and the semi-arch shaped bushes configured to be inserted between the rotating shaft part of the rotating link and the sun gear; a spring member configured to be mounted in a long-hole concave groove formed on an outer circumferential surface of the bushes; and a bush protrusion formed at one end of the bushes to be inserted into a protrusion insertion groove formed on the rotating link. semi-arch shaped bushes formed in a pair to face each other and the semi-arch shaped bushes configured to be inserted between the rotating shaft part of the rotating link and the sun gear; a spring member configured to be mounted in a long-hole concave groove formed on an outer circumferential surface of the bushes; and a bush protrusion formed at one end of the bushes to be inserted into a protrusion insertion groove formed on the rotating link.