THERMAL FLOWMETER

PROBLEM TO BE SOLVED: To reduce measurement errors of a mass flow rate and a detection frequency of abnormal values in a thermal flowmeter by reducing an amount of contaminants in an air current within piping, which reach a sensor part.SOLUTION: The thermal flowmeter comprises a measurement unit 310 provided with an auxiliary passage 2 for taking in a part of an air current 30 flowing in a main passage 124 and a flow detection part 602 for measuring a flow rate of the air current flowing in the auxiliary passage 2. A shape of the measurement unit 310 projected on an orthogonal plane orthogonal to the flow direction of the air current 30 has a length defined in a first direction on the orthogonal plane and a thickness defined in a second direction perpendicular to the first direction on the orthogonal plane, and the thickness is larger than the length. An expanded part 2w having a larger passage width than on the upstream side is formed in an auxiliary passage part above the flow detection part 602. The expanded part 2w is formed on the side of a wall surface partitioning the auxiliary flow passage 2 in the first direction.