To provide a constant temperature driving circuit of a TCD sensor wherein output is stabilized when power supply is started and power supply capacity is large.

A transistor Q1 which supplies driving current I from a power supply +V to Wheatstone bridge is provided, and the transistor Q1 is controlled by the output ΔV of an operation amplifier 3. When power supply is started, by an input potential setting means 5 consisting of resistors R10 and R11, a base potential VB which is larger than base-emitter voltage VBE of the transistor Q1 is inputted, so the output impedance of a transistor drops so that the current I is applied to the Wheatstone bridge.