PURPOSE: To enable the hammer bank of a printer to be used out of the unstable action frequency region of printing hammers by varying the speeds of the shuttle motor to move the hammer bank in a rocking manner according to the turning angles.

CONSTITUTION: In case where a hammer bank 1 is moved at equal speeds, the turning speed of a shuttle motor is represented by the formula, where ω is angular speed of shuttle motor, ω0 is angular speed of shuttle motor when θ=0, and θ is turning angle of shuttle motor, provided that θ=0 when shuttle action is on the center. When the displacement of shuttle motor is within the range x1Wx14, the speed of shuttle motor is varied as represented by the formula, and when the displacement of shuttle motor exceeds printing ranges, the shuttle motor is turned at constant speed. The hammer bank 1 can thus be moved at equal speed within the printing range on the basis of such a relationship between the turning angle and angular speed of the shuttle motor, enabling the maximum responding frequency of printing hammers to be used within printing range all the time. The speeds of printing process can thus be increased and the hammer bank can be used by avoiding the unstable action frequency region of hammers.