A normally closed three-port valve includes a valve member urged from a supply port side toward a main valve seat of a valve body in a valve closing direction and opened by a valve opening rod having a discharge channel. A back pressure chamber at a back of the valve member communicates with a channel on an output port side through a vent hole balancing fluid pressures acting on front and back surfaces of the valve member. The valve member includes a main body, a seat member, a securing member, and a pressing frame The seat member includes an elastic discoid sealing member, main and discharge seat surfaces, and communication holes open at a region between the main and discharge seat surfaces forming the vent hole. The securing member secures the seat member to the valve member main body. The pressing frame is at the circumference of the seat member.