PURPOSE: To provide a mass flow rate sensor by which highly accurate measurement of flow rate can be carried out by reducing effect of environmental temperature on the sensor provided on a pipeline.

CONSTITUTION: In the mass flow rate sensor which is provided with a heater 2 on the surface of a pipeline 1 through which a fluid G flows, and a first temperature sensor 9 and a second temperature sensor 10 separately on the surface of the pipeline 1 on the upstream side and on the downstream side from the heater 2, heat radiators 3 and 4 are arranged respectively on the upstream side from the first temperature sensor 9 and on the downstream side from the second temperature sensor 10 of the pipeline 1 to always maintain the temperature of the heat radiators 3 and 4 at a fixed value.