Functional dual-purpose sensor

The invention provides a functional dual-purpose sensor which can achieve functions of a flow velocity sensor and a thermal conductivity sensor in the same circuit and can switch the various functions to be individually used. The functional dual-purpose sensor is provided with a heater drive circuit, a constant-voltage circuit outputting a constant voltage, and a switch; the heater drive circuit at least is provided with a heater element a voltage between terminals changes according to the fluid flow rate, a temperature sensor the voltage between the terminals changes corresponding to fluid ambient temperature, and an operational amplifier outputting a measurement potential correspondingly taken as a difference of a heater potential and a reference potential of the voltage between the terminals of the heater element; and whether the voltage between the terminals of the temperature sensor or the constant voltage is taken as the reference potential to be supplied to the operational amplifier is switched by the switch. When the voltage between the terminals of the temperature sensor is taken as the reference potential to be supplied, the fluid velocity sensor taken as the measurement potential and changing according to the fluid flow rate acts; and, when the constant voltage is taken as the reference potential to be supplied, the thermal conductivity sensor taken as the measurement potential and changing according to the fluid thermal conductivity acts.