An ejector comprises a body, a nozzle, a needle, a diffuser which draws in a second fluid using negative pressure caused by ejection of a first fluid from the nozzle and mixes the first and second fluids together, first and second diaphragms which allows the nozzle to shift in an axial direction with respect to the needle, and a first fluid chamber which is supplied with the first fluid. A valve in which a valve body contacts and separates from a valve seat according to the shifting action of the nozzle is formed by providing either the nozzle or the needle with the valve body and providing the other with the valve seat in the first fluid chamber. A back pressure chamber connecting to the first fluid chamber via the valve is provided between a trunk portion of the nozzle and a basal part of the needle.