A circuit board is located in a flow rate measurement passage of a housing. The flow rate measurement passage has: a first inner surface that is located at one side of the flow rate measurement passage where a primary lateral surface of the housing is placed; and a second inner surface that is located at another side of the flow rate measurement passage where a secondary lateral surface of the housing is placed. A flow rate sensing device is installed to one side of the circuit board where the first inner surface is placed. A distance, which is measured from the circuit board to the first inner surface in a plate thickness direction of the circuit board, is larger than a distance, which is measured from the circuit board to the second inner surface in the plate thickness direction of the circuit board.