PURPOSE: To enable detection of flow rate without being affected by a heat constant of fluid, after exciting thermally a heating element set in the specimen fluid, dissipation of heat, by measuring time until temperature drops to the specified temperature from the time when the temperature of the element passes through the No.1 specified temperature.

CONSTITUTION: When an electric source on-pulse PWO is received by an OR- gate circuit 44, a detecting circuit unit 1 clears circuits 35, 50, 53, and heats a heating elements QH. The unitl judges its temperature whether it is above or below the reference temperature TA+T3 by a comparing circuit 36. At a point where a temperature detecting signal TDS passes consecutively the reference temperatures REF1WREF3, the circuit 1 stops heating of the element QH, through circuit 35 and switch circuits 24, 31 and let the heat of the element QH to be given to the wind. The circuit 1 detects time from passage of the temperature through TA+T2 to drop to TA+T1 by a reciprocal counter circuit 53 and from the result, a wind speed detecting output VEL is calculated.