A gas measurement adapter comprising: 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 , wherein the window portion is thinner than the flow tube portion and is molded integrally with the flow tube portion , wherein an inner surface of a window portion defines a portion of a ventilation passage through which the gas passes inside the gas measurement adapter , and characterized in that i) 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 , the inner surface of the window portion being configured such that an outer perimeter of the effective portion is contiguously coplanar with the outer peripheral portion , wherein the inner surface of the window portion is contiguously coplanar with an inner surface of the flow tube portion , or ii) an outer perimeter of the window portion on the inner surface of the window portion is contiguously coplanar with a portion of the flow tube portion bordering the outer perimeter of the window portion . 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 , wherein the window portion is thinner than the flow tube portion and is molded integrally with the flow tube portion , wherein an inner surface of a window portion defines a portion of a ventilation passage through which the gas passes inside the gas measurement adapter , and characterized in that i) 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 , the inner surface of the window portion being configured such that an outer perimeter of the effective portion is contiguously coplanar with the outer peripheral portion , wherein the inner surface of the window portion is contiguously coplanar with an inner surface of the flow tube portion , or ii) an outer perimeter of the window portion on the inner surface of the window portion is contiguously coplanar with a portion of the flow tube portion bordering the outer perimeter of the window portion . i) 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 , the inner surface of the window portion being configured such that an outer perimeter of the effective portion is contiguously coplanar with the outer peripheral portion , wherein the inner surface of the window portion is contiguously coplanar with an inner surface of the flow tube portion , or ii) an outer perimeter of the window portion on the inner surface of the window portion is contiguously coplanar with a portion of the flow tube portion bordering the outer perimeter of the window portion . <EOS>