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 (7, 24, 31, 40, 73) which is connected to the outlet pipe and arranged inside the device body; a flow rate measuring unit (9, 9a) which is connected to the connecting pipe (7, 24, 31, 40, 73) and measures a flow rate of the fluid to be measured which flows in the flow rate measuring unit (9, 9a); a flow passage member (10, 10a) which is connected to the connecting pipe (7, 24, 31, 40, 73) and has a flow passage shape identical to a shape of the flow rate measuring unit (9, 9a); and a support member (11, 26, 33, 42, 59, 76) by which the flow rate measuring unit (9, 9a) and the flow passage member (10, 10a) are joined to each other; characterized in that: the flow rate measuring unit (9, 9a) is an ultrasonic flow rate measuring unit (9, 9a); the device body includes one case and the other case, their peripheries are air-tightly sealed; the support member (11, 26, 33, 42, 59, 76) connects the ultrasonic flow rate measuring unit (9, 9a) and the flow passage member (10, 10a) at position in the vicinity of the connecting pipe (7, 24, 31, 40, 73); the outlet pipe is connected to the connecting pipe (7, 24, 31, 40, 73) in the inside of the device body; the ultrasonic flow rate measuring unit (9, 9a), the flow passage member (10, 10a), the connecting pipe (7, 24, 31, 40, 73) and the support member (11, 26, 33, 42, 59, 76) 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 (7, 24, 31, 40, 73) which is connected to the outlet pipe and arranged inside the device body; a flow rate measuring unit (9, 9a) which is connected to the connecting pipe (7, 24, 31, 40, 73) and measures a flow rate of the fluid to be measured which flows in the flow rate measuring unit (9, 9a); a flow passage member (10, 10a) which is connected to the connecting pipe (7, 24, 31, 40, 73) and has a flow passage shape identical to a shape of the flow rate measuring unit (9, 9a); and a support member (11, 26, 33, 42, 59, 76) by which the flow rate measuring unit (9, 9a) and the flow passage member (10, 10a) are joined to each other; characterized in that: the flow rate measuring unit (9, 9a) is an ultrasonic flow rate measuring unit (9, 9a); the device body includes one case and the other case, their peripheries are air-tightly sealed; the support member (11, 26, 33, 42, 59, 76) connects the ultrasonic flow rate measuring unit (9, 9a) and the flow passage member (10, 10a) at position in the vicinity of the connecting pipe (7, 24, 31, 40, 73); the outlet pipe is connected to the connecting pipe (7, 24, 31, 40, 73) in the inside of the device body; the ultrasonic flow rate measuring unit (9, 9a), the flow passage member (10, 10a), the connecting pipe (7, 24, 31, 40, 73) and the support member (11, 26, 33, 42, 59, 76) are integrally formed with each other and formed into the device body. the flow rate measuring unit (9, 9a) is an ultrasonic flow rate measuring unit (9, 9a); the device body includes one case and the other case, their peripheries are air-tightly sealed; the support member (11, 26, 33, 42, 59, 76) connects the ultrasonic flow rate measuring unit (9, 9a) and the flow passage member (10, 10a) at position in the vicinity of the connecting pipe (7, 24, 31, 40, 73); the outlet pipe is connected to the connecting pipe (7, 24, 31, 40, 73) in the inside of the device body; the ultrasonic flow rate measuring unit (9, 9a), the flow passage member (10, 10a), the connecting pipe (7, 24, 31, 40, 73) and the support member (11, 26, 33, 42, 59, 76) are integrally formed with each other and formed into the device body.