PURPOSE: To easily compensate an angular error during introduction of a plate material by providing a parameter registering section for registering an offset value of a rotational angle of a rotary pad and a clamping angle offset means in a plate material rotating device for a plate material machining device such as a bender or the like.

CONSTITUTION: A work clamper 14 clamps a plate material W which has been positioned by a suitable means, with the use of a pair of rotary pads 12, 13. During this clamping operation, a clamping angle offset means 33 causes a servomotor 22 to rotate the rotary pad 13 by an offset value (a) registered in a parameter registering section 32 so as to offset the angle for clamping the plate material W between the rotary pads 12, 13. When plate material W is rotated and indexed, the rotary pad 13 is rotated in accordance with an instructed value Tθ given by a machining program 29. Accordingly, the rotary pad 13 is rotated by an angle which is given by adding the offset value (A) to the instructed value Tθ, and accordingly, the compensation is made by the offset value (a).