PURPOSE: To enhance flow amount measuring accuracy as high as possible by enlarging a dynamic range, by dropping the output voltage from a bridge circuit by predetermined voltage by a shift-down circuit and subsequently amplifying the dropped voltage to input the same to an A/D converter.

CONSTITUTION: The current supplied from a control circuit corresponding to the flow amount of a fluid is flowing to the resistor R3 connected to a heat sensitive resistor RH in series and the terminal voltage corresponding to said current is inputted to a shift-down circuit S. The shift-down circuit S outputs voltage obtained by subtracting predetermined voltage from the terminal voltage of the resistor R3 and this output voltage is inputted to an amplifying circuit A which in turn amplifies this input voltage to output the same. The output voltage from the amplifying circuit A is inputted to an A/D converter 1 to be converted to a digital value which is, in turn, inputted to an operation circuit (microcomputer) 2. The operation circuit 2 operates the flow amount of the fluid on the basis of the inputted digital value.