A valve drive device includes a valve seat plate in which an inflow port and an outflow port for fluid are formed, a sealing case covering the front face side of the valve seat plate, an inflow pipe and an outflow pipe which are fixed in the rear face side of the valve seat plate so as to be in communication with the inflow port and the outflow port, and a valve element which slides on an area where the outflow port is formed on the front face side of the valve seat plate to open or close the outflow port. The valve seat plate includes a first plate member which constructs an area where the outflow port is formed, a second plate member which is thinner than the first plate member and is jointed with the first plate member, and a deformation preventing member which is attached on the rear face of the second plate member and prevents the second plate member from being deformed due to an internal-pressure in a sealed space constructed by the sealing case and the valve seat plate.