In a hydraulic pressure control system for an engine cylinder cutoff device through which a first group of cylinders are cut off with at least intake valves of engine valves included in the first group kept inactive, while a second group of cylinders are working, the system includes a valve mode switching mechanism being responsive to supply oil pressure for switching an operating mode of at least an intake valve of engine valves of each of the first group of engine cylinders from active to inactive. Also provided is a hydraulic pressure control valve regulating the supply oil pressure. A control unit incorporated in the system sets the supply oil pressure to a predetermined maximum pressure when initiating the inactive mode, and holds the supply oil pressure at a predetermined oil pressure lower than the predetermined maximum pressure before the inactive mode is released.