A valve device capable of precisely adjusting a flow rate includes: an operating member for operating a diaphragm and provided movably between a closed position at which diaphragm closes a flow path and an open position at which diaphragm opens the flow path; a main actuator that receives pressure from a supplied drive fluid and moves the operating member to the open position or the closed position; an adjusting actuator for adjusting the position of the operating member positioned in the open position by using a passive element which expands and contracts in response to a given input signal; a position detecting mechanism for detecting the position of the operating member with respect to a valve body; and an origin position determining unit that uses a valve closed state in which the diaphragm contacts to valve seat to determine an origin position of the position detecting mechanism.