PURPOSE: To produce a small device to reduce plate thickness while a plate material is advanced continuously by reducing the plate thickness locally in the width direction of the plate material and moving this position in the direction of the plate width to reduce the weight and torque.

CONSTITUTION: The sum of the diameters of a pair of rollers 1A, 1B is reduced gradually from the central part toward the end part. At least, one roller, for example, 1A is supported by the neck parts 2A, 2B to a housing through a pair of oscillating actuators, for example, a pair of hydraulic cylinders 3A, 3B so that it can seesaw. From a source 4 of oscillation, a hydraulic flow is turned toward the directions of arrows 5A, 5B, cylinders 3A, 3B are moved in the directions of arrows 6A, 6B, a roller 1A is seesawed in the directions of arrows 7A, 7B, a working point of the plate material is moved in the width direction of an arrow 8. When the reduction of the plate thickness spreads as far as the end of the plate material, the hydraulic flow from the source 4 is turned to make up a movement in the direction opposite to the above.