A valveless micropump for transforming a pressure variation of fluid in a volume variation chamber (8) into an oscillatory flow of fluid (10) by means of a piezoelectric element (7), making the fluid (10) to flow smoothly in channels (5, 6). A channel (2) is formed as an asymmetric diffuser-shaped channel (12) having a narrow channel (5) on the diffuser inlet (3) side and a wide channel (6) on the diffuser outlet (4) side, and the volume variation chamber (8) provided with the piezoelectric element (7) is communicated with the narrow channel (5). Vibration generated by applying a voltage to the piezoelectric element (7) causes a pressure variation of the fluid in the volume variation chamber (8) to generate a nozzle flow. The fluid (10) is made to flow smoothly from the wide channel (6) to the narrow channel (5) by the nozzle flow.