PURPOSE: To make it possible to reduce a zero drift range to a large extent, by locally changing the surface treatment of a sensor pipe or cases facing the sensor pipe.

CONSTITUTION: A fluid to be measured flows in a sensor pipe 4. The sensor pipe 4 is heated by electric means 5 and 6. A means, which detects the difference in temperature of the sensor pipe 4 at its upstream side and its downstream side, is provided through relay terminals 12 and 13. They are accommodated in cases 10 and 20. The surface treatment of the sensor pipe 4 or the cases 10 and 20 facing the sensor pipe 4 is locally changed. Thus a zero drift range is reduced to a large extent. As the surface treatment, e.g., a black paint is applied to the sensor pipe 4 or the part of a groove 11 in the inside of the cases 10 and 20.