To provide an auxiliary apparatus for mounting a press plate of a printer capable of simplifying a constitution and capable of stably holding the plate by accurately positioning the plate to enhance a printing accuracy and eliminating a skill of positioning the plate.

After a positioning cutout formed at a gripper side end of a press plate is filled with a positioning pin 11 fixed to a gripper side clamp base 51 and aligned, a gripper side clamp 52 is closed to fix the gripper side end of the plate. Since a gap L1 between a plate guide 15 disposed near the pin 11 and the base 51 is an interval slightly larger than a thickness of the plate, the gripper side end of the plate an be easily inserted. An opposite side to the gripper end of the plate is brought into contact with a support member, and the gripper side end of the plate can be inserted into the gap L1 by utilizing a restoring force caused by a slack of the plate.