PURPOSE: To measure temp. with high reliability at high temp., by calculating temp. from the measuring result of the pressure difference between both ends of the throttled part of a hydraulic fluid flow passage supplying a hydraulic fluid in a definite mass flow amount.

CONSTITUTION: A high pressure hydraulic fluid is supplied from a hydraulic fluid supply source. The hydraulic fluid is supplied to the flow passage 3 of a sensor 1 in a definite mass flow amount Q by a mass flow controller 8 and discharged to the atmosphere from an exhaust port 1a through a capillary tube 2. At this time, pressure loss ΔP is generated in the part of the capillary tube 2 and shown by formula ΔP=8Qlμ(T)/{π(d/2)4} [wherein T is temp., d is the inner diameter of the capillary tube 2 at the temp. T, l is a length and μ(T) is dynamic viscosity]. When a gas, of which the state change due to temp. is known, such as Ar gas is used as the hydraulic fluid, ΔP, that is, the difference pressure between the pressure P1 in the flow passage 3 and the pressure difference P2 at a hydraulic fluid discharge port 1a is measured to make it possible to determine the temp. T.