PURPOSE: To enable a rack and a planetary gear to be securely engaged and disengaged from each other and enable the drag at the time of disengagement to be reduced, by a construction wherein a straight line perpendicular to a meshing tooth surface of a rack for meshing with a planetary gear lines on the rack side of the center of a sun gear and forms a specified angle with the pitch line of the rack.

CONSTITUTION: A planetary gear 11 is located on the front side of a sun gear 10 as viewed in a printing direction, the perpendicular X to a meshing tooth surface of a rack 12 for meshing with the planetary gear 11 lies on the rack 12 side of the center 8 of the sun gear 10, with a spacing S therebetween, and the angle θ between the perpendicular X and the pitch line P of the rack is set to be not less than 21°. The planetary gear 11 is provided with an involute tooth profile, while the rack 12 is provided with an asymmetric tooth profile in which the meshing tooth surface is gently sloped while the right side surface is steeply sloped. With this construction, the force of a spring 14 can be reduced, the drag at the time of disengaging the planetary gear 11 can be reduced, and a disengaging driving force W from a driving rod 13 can be reduced.