A gas flowmeter comprising: a device body which air-tightly accommodates a fluid to be measured; an inlet pipe through which the fluid to be measured is made to flow into the device body ; an outlet pipe through which the fluid to be measured is made to flow out from the device body ; a connecting pipe which is connected to the outlet pipe and arranged inside the device body ; a flow rate measuring unit which is connected to the connecting pipe and measures a flow rate of the fluid to be measured which flows in the flow rate measuring unit ; a flow passage member which is connected to the connecting pipe and has a flow passage shape identical to a shape of the flow rate measuring unit ; and a support member by which the flow rate measuring unit and the flow passage member are joined to each other; characterized in that : the flow rate measuring unit is an ultrasonic flow rate measuring unit ; the device body includes one case and the other case , their peripheries are air-tightly sealed; the support member connects the ultrasonic flow rate measuring unit and the flow passage member at position in the vicinity of the connecting pipe ; the outlet pipe is connected to the connecting pipe in the inside of the device body ; the ultrasonic flow rate measuring unit , the flow passage member , the connecting pipe and the support member are integrally formed with each other and formed into the device body . a device body which air-tightly accommodates a fluid to be measured; an inlet pipe through which the fluid to be measured is made to flow into the device body ; an outlet pipe through which the fluid to be measured is made to flow out from the device body ; a connecting pipe which is connected to the outlet pipe and arranged inside the device body ; a flow rate measuring unit which is connected to the connecting pipe and measures a flow rate of the fluid to be measured which flows in the flow rate measuring unit ; a flow passage member which is connected to the connecting pipe and has a flow passage shape identical to a shape of the flow rate measuring unit ; and a support member by which the flow rate measuring unit and the flow passage member are joined to each other; characterized in that : the flow rate measuring unit is an ultrasonic flow rate measuring unit ; the device body includes one case and the other case , their peripheries are air-tightly sealed; the support member connects the ultrasonic flow rate measuring unit and the flow passage member at position in the vicinity of the connecting pipe ; the outlet pipe is connected to the connecting pipe in the inside of the device body ; the ultrasonic flow rate measuring unit , the flow passage member , the connecting pipe and the support member are integrally formed with each other and formed into the device body . the flow rate measuring unit is an ultrasonic flow rate measuring unit ; the device body includes one case and the other case , their peripheries are air-tightly sealed; the support member connects the ultrasonic flow rate measuring unit and the flow passage member at position in the vicinity of the connecting pipe ; the outlet pipe is connected to the connecting pipe in the inside of the device body ; the ultrasonic flow rate measuring unit , the flow passage member , the connecting pipe and the support member are integrally formed with each other and formed into the device body . <EOS>