PURPOSE: To remove malfunctions due to noises and improve prcision in a supersonic flowmeter by restricting time width of gate signals only signal sound wave can pass through.

CONSTITUTION: There are provided a receiving circuit 17A or 17B which receives supersonic waves emitted from one oscillator to a measured fluid at the other oscillator, a counter 10 which generates an output when the output of an oscillator 9A or 9B is counted and reaches a prescribed value and a time difference conversion circuit 7 which requires the time difference between the output of this counter 10 and that of the receiving circuit 17A or 17B. Output frequencies of the oscillator are changed according to the output of this time difference conversion circuit 7 and the outflow is measured but when the integrated output of the oscillator reaches a prescribed value the receiving circuit receives supersonic waves from the oscillator for a prescribed time interval.