To measure from a small flow rate to a large flow rate with one microflow-sensor by measuring the flow rate of fluid based on the detected measured temperature signals of temperature measuring elements arranged on the upstream side and the downstream side sandwiching a heater element.

Two temperature measuring elements 32, 33 sandwiching a heater element 34 are arranged on a diaphragm to form a microflowsensor 3, and it is arranged on the inner wall of a flow passage 17. Simultaneously with beginning of measurement, fixed constant voltage is output from an electric power source 5, for a time (for example 10-20 mm/sec) corresponding to the pulse width T set by a drive pulse width variable means 7, supplied to the heater element 34, and fluid in the flow passage 17 is heated. Fluid temperature is measured by the temperature measuring elements 32, 33 arranged respectively on the upstream side and the downstream side of the heater element 17, and sent to a flow computing part 9. The flow computing part 9 computes fluid flow speed from the differential amplifying signal of the measured temperature signals from both temperature measuring elements 32, 33, and the flow of fluid in the flow passage 17 is computed based on the flow speed.