A channel is formed as an asymmetric diffuser-shaped channel having a narrow channel on a diffuser inlet side and a wide channel on a diffuser outlet side. The narrow channel is communicated with a variable volume chamber which is provided therein with a piezoelectric element. Vibration generated by actuation of the piezoelectric element causes a pressure variation of a fluid in the variable volume chamber to generate a nozzle flow which in turn causes a smooth flow of the fluid from the wide channel to the narrow channel.