To regulate the flow rate of a fluid, and form a control valve mechanism having a high reliability at low cost by switch-controlling a 4 port-3 position solenoid switching valve so as to match a setting signal outputted by valve opening setting means and a detecting signal with each other.

An opening control device comprises a 4 port-3 position solenoid switching valve 12A, an auxiliary hydraulic circuit provided in parallel with a main hydraulic circuit, valve opening detecting means 14, valve control means 15 for switching and controlling the valve 12A, a valve opening supporting system 17, a three way switching valve 18 for switching a hydraulic source, and a three way switching valve 19 for switching an oil tank. A setting signal outputted by the means 14 and a detecting signal outputted by valve opening detecting means 13 are compared with each other, an electric output according to its difference is generated from an output circuit, either one of solenoid coils in the valve 12A is selected, the electric output is applied. When both signals are matched with each other so as to eliminate its difference, the electric output is disconnected.