A gas measurement adapter includes a flow tube portion configured to allow gas to pass through the flow tube portion, and a window portion configured to allow measurement light for measuring a component of the gas passing through the flow tube portion to pass through the window portion. The window portion is thinner than the flow tube portion and is molded integrally with the flow tube portion. The window portion comprises an effective portion configured to allow the measurement light to pass through the effective portion and an outer peripheral portion provided on an outer periphery of the effective portion. An inner surface of the window portion is configured such that an outer perimeter of the effective portion is contiguously coplanar with the outer peripheral portion.