PURPOSE: To correct the curved curling habit of a sheet material, by passing the sheet material between a large number of rolls arranged in a zigzag pattern within a thermostatic tank, of which the atmospheric temp. was raised, so as to be rotatable at low torque.

CONSTITUTION: A large number of decurling roll groups 5', 5", 5''' are arranged in a thermostatic tank 4 of which the atmospheric temp. was made constant at about 60°C and a roll sheet comprising a plastic elastomer material is passed between the roll groups in the thermostatic tank 4 from a sheet material roll 1 through guide rolls 2, 3 and taken out of the tank 4 to be drawn out by a nip roll 8 through a guide roll 3'. By this method, the curling habit of said roll sheet is corrected and a flat sheet can be obtained.