PURPOSE: To improve the consumption unit of roll and to prevent the lowering of operating efficiency by detecting the number of revolutions of a work roll and the load of bearings and releasing the hydraulic valve of a rolling reduction device when the number of revolutions and its lowering rate reach the prescribed conditions.

CONSTITUTION: The number of revolutions N of a work roll 1 is detected by a pulse generator 12 connected to a mill motor 11. A rolling load F is detected by a load cell 13 placed at the upper part of a mill housing. A hydraulic servovalve 9 is released; when a continued time of revolution, at the condition that the number of revolutions N is kept at N≤Na with respect to the allowable minimum number of revolutions Na, exceeds the allowable duration of time, or an oil-film bearing 17 supports a load larger than its allowable value for a period longer than a limited duration of time, or further the lowering rate of the number of revolutions of roll 1 i.e. -dN/dt, lowers to a ≤ limit of lowering rate α. That is, a microcomputor 14 performs a comparison operation with the prescribed conditions based on the inputted number of revolutions N and rolling load F, to release a cylinder 8 for protecting the roll 1.