A power transmission apparatus comprising: a rectilinear rack allowing a pinion to perform a rectilinear motion by interacting with the pinion; a curvilinear rack allowing the pinion to perform a curvilinear motion by interacting with the pinion; and a rectilinear-curvilinear conversion rack connected to the rectilinear rack and the curvilinear rack between the rectilinear rack and the curvilinear rack, and converting the rectilinear motion and the curvilinear motion of the pinion, characterized in that: the rectilinear-curvilinear conversion rack comprises: a rectilinear part having a rectilinear tooth and connected to the rectilinear rack; and a rectilinear-curvilinear conversion part having a rectilinear-curvilinear conversion tooth and having one side integrally formed into the rectilinear part and other side connected to the curvilinear rack, wherein the rectilinear tooth is provided in a plural number on the rectilinear part, wherein the rectilinear-curvilinear conversion tooth provided in the rectilinear-curvilinear conversion part is one or two, wherein tooth shapes of opposite side surfaces of the rectilinear-curvilinear conversion tooth are asymmetrically formed with respect to a centerline, wherein a curvature of a first tooth shape formed at one side surface of the rectilinear-curvilinear conversion tooth matches a curvature of tooth shapes of teeth formed at the curvilinear rack, wherein a curvature of a second tooth shape formed at other side surface of the rectilinear-curvilinear conversion tooth matches a curvature of tooth shaped of teeth formed at the rectilinear rack. a rectilinear rack allowing a pinion to perform a rectilinear motion by interacting with the pinion; a curvilinear rack allowing the pinion to perform a curvilinear motion by interacting with the pinion; and a rectilinear-curvilinear conversion rack connected to the rectilinear rack and the curvilinear rack between the rectilinear rack and the curvilinear rack, and converting the rectilinear motion and the curvilinear motion of the pinion, characterized in that: the rectilinear-curvilinear conversion rack comprises: a rectilinear part having a rectilinear tooth and connected to the rectilinear rack; and a rectilinear-curvilinear conversion part having a rectilinear-curvilinear conversion tooth and having one side integrally formed into the rectilinear part and other side connected to the curvilinear rack, wherein the rectilinear tooth is provided in a plural number on the rectilinear part, wherein the rectilinear-curvilinear conversion tooth provided in the rectilinear-curvilinear conversion part is one or two, wherein tooth shapes of opposite side surfaces of the rectilinear-curvilinear conversion tooth are asymmetrically formed with respect to a centerline, wherein a curvature of a first tooth shape formed at one side surface of the rectilinear-curvilinear conversion tooth matches a curvature of tooth shapes of teeth formed at the curvilinear rack, wherein a curvature of a second tooth shape formed at other side surface of the rectilinear-curvilinear conversion tooth matches a curvature of tooth shaped of teeth formed at the rectilinear rack. the rectilinear-curvilinear conversion rack comprises: a rectilinear part having a rectilinear tooth and connected to the rectilinear rack; and a rectilinear-curvilinear conversion part having a rectilinear-curvilinear conversion tooth and having one side integrally formed into the rectilinear part and other side connected to the curvilinear rack, a rectilinear part having a rectilinear tooth and connected to the rectilinear rack; and a rectilinear-curvilinear conversion part having a rectilinear-curvilinear conversion tooth and having one side integrally formed into the rectilinear part and other side connected to the curvilinear rack, wherein the rectilinear tooth is provided in a plural number on the rectilinear part, wherein the rectilinear-curvilinear conversion tooth provided in the rectilinear-curvilinear conversion part is one or two, wherein tooth shapes of opposite side surfaces of the rectilinear-curvilinear conversion tooth are asymmetrically formed with respect to a centerline, wherein a curvature of a first tooth shape formed at one side surface of the rectilinear-curvilinear conversion tooth matches a curvature of tooth shapes of teeth formed at the curvilinear rack, wherein a curvature of a second tooth shape formed at other side surface of the rectilinear-curvilinear conversion tooth matches a curvature of tooth shaped of teeth formed at the rectilinear rack.