To secure safety of an operator, and to restrain a facility cost from increasing and workability from getting low in a plate replacing device and method.

This plate replacing device includes plate replacing cassettes 36a, 36b capable of holding press plates 100a, 100b, and inclined to approach tip parts to plate cylinders 22a, 22b, inclined position detecting parts 71a, 71b for detecting inclined positions of the plate replacing cassettes 36a, 36b, plate replacing work starting switch 73 for commanding a plate replacing operation between the plate replacing cassettes 36a, 36b and the plate cylinders 22a, 22b, and a controller 9 for receiving a command from the plate replacing work starting switch 73, to carry out plate replacing work, when the inclined position detecting parts 71a, 71b detect the inclined positions of the plate replacing cassettes 36a, 36b.