PURPOSE: To ensure the prevention of a water hammer phenomenon, and the possibility of the sequential speed of two liquids by arranging both open/close valves according to such a positional relationship as disabling the opening or closing operation of either valve in a sequence other than specified, due to the interference of a lever and a valve spindle, when the valves are in a specific open or close condition.

CONSTITUTION: When both a main open/close valve 3 and a sub-open/close valve 4 are wide open for allowing a maximum flowrate. Also, when the sub- open/close valve 4 is open, the main open/close valve 3 can be freely opened and closed, thereby changing a flowrate from a minimum level to a maximum level. A pipe line in operation is used in the aforesaid state. Also, when a valve spindle 5 is being lifted, a lever 6 comes in contact therewith. The closing operation of the sub-open/close valve 4 is disabled, and it is always required to close the main open/close valve 3 before closing the sub-open/close valve 4. In this case, when the main open/close valve 3 is totally closed, the sub-open/ close valve 4 is still open, and there is a certain flow through the valve 4. No water hammer phenomenon, therefore, does not appear.