An ejector and a fuel cell system using the ejector which can improve a control of an ejection pressure of a fluid.The ejector 50 includes a body 60, a nozzle 80, a needle 70, a diffuser 90 which sucks a second fluid by a negative pressure generated by a first fluid ejected from the nozzle 80, and a first, a second and a third diaphragms 100, 110, 120 which are movable in the axial direction against the needle 70. The first diaphragm 100 and the second diaphragm 110 have the same effective area, and an effective area of the third diaphragm 120 is different from those of the first diaphragm 100 and the second diaphragm 110.