PURPOSE: To make etching treatment uniform and efficient by providing a heat treating part to the adjacency of the down stream of an etching part, and performing etching while heat treating an offset master.

CONSTITUTION: A master 2 has a plate surface 2a, wherein a picture is formed by a plate manufacturing machine 1. The master 2 is sent to the etching part 5 along an inserting guide plate 4 with the surface of the plate surface 2a on the upper side and conveyed by rotary etching rollers 7a and 7b in which etching liquid 6 is sucked. The etching liquid is applied on the plate surface during the conveyance. With the etching treatment being progressed on the plate surface of the master 2, it is conveyed into the heat treating part 9 and heated by a heat source 11. By this heat treatment, the etching reaction between the etching liquid on the plate surface of the master 2 and the non-picute part of the master is funther expedited, and the uniform and perfect etching treatment is finished.