A housing has a front surface, a rear surface, and side surface connecting the front surface with the rear surface. The housing is formed by injection molding. A first sub-passage is formed in the housing and communicates a first sub-passage inlet formed in the front surface with a first sub-passage outlet formed in the rear surface. A second sub-passage is formed in the housing and communicates a second sub-passage inlet formed in a midway portion of the first sub-passage with a second sub-passage outlet at a position different from the first sub-passage outlet. A flow rate detection unit is provided in the second sub-passage. A mold parting mark is formed in the rear surface of the housing at a position away from an inner opening edge of the first sub-passage outlet.