A flow rate measuring device includes a housing having a cross-section perpendicular to a Y-direction and taken at a position of an outlet of the housing. The cross-section includes a widest portion having a maximum width along a Z-direction. The cross-section has a width gradually decreasing from the widest portion toward an upstream side end and has a width gradually decreasing from the widest portion toward a downstream side end. A width of the housing at a middle position of the housing in an upstream area is defined as a width W1. A width at a middle position in a downstream area is defined as a width W2. A length of the housing in the upstream area is defined as a length L1. A length in the downstream area is defined as a length L2. W1>W2 and W1/L1>W2/L2.