A control unit 65 of a cardboard sheet manufacturing apparatus 10 can execute a steady operation in which a corrugated roll 17A is rotated at a first set velocity V1, then execute a deceleration operation in which the corrugated roll 17A is decelerated to a second set velocity V2, and thereafter execute an acceleration operation in which the corrugated roll 17A is accelerated to a third set velocity V3. In the steady operation, the control unit 65 places an open/close valve 60A in a closed state, and supplies steam to a hollow part 17b at a first set pressure P1 using a pressure adjusting valve 55. In the deceleration operation, the control unit 65 maintains the states of the open/close valve 60A and the pressure adjusting valve 55. In the acceleration operation, the control unit 65 switches the open/close valve 60A from the closed state to an open state, and adjusts the pressure adjusting valve 55 such that steam is supplied to the hollow part 17b at a second set pressure P2.
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