To provide a fluid flow velocity measuring instrument used not only for performing efficient measurement with a measurement down period eliminated but for performing measurement even if flow velocity is zero.

A heater 3 starts heat generation by its power application to raise the temperature of the heater. Heat transfer takes place in temperature sensing elements 1 and 2. An output voltage 10 owing to a temperature rise of the sensing element 1 reaching V2 causes a trigger signal to be output from a comparator 6. An output of a logic element 8 becomes Low to cause a heater control circuit 9 to stop the power application to the heater 3. Because of this, heat transferring to the temperature sensing elements 1 and 2 decreases. An output voltage of the sensing element 2 reaching V1 causes a trigger signal to be output from a comparator 7. The output of the logic element 8 becomes High to start the power application to the heater 3. This operation is repeated to cause output pulses to be output from the logic element 8. Pulse intervals T1 and T2 are measured of these output pulses.