To accurately measure the exhaust gas flow rate after enough and efficiently mixing the exhaust gas with a trace gas.

In a pipe 1 a nozzle 28 is provided right angles to a flowing direction of an exhaust gas G, with forming gaps between the inner wall of the pipe 1 and the ends of the nozzle 28, the nozzle 28 is rotatably supported on the top end 3 of an entrance pipe 2, and a plurality of gas jet holes 7 communicating with the entrance pipe 2 are formed in the length direction of the nozzle 28 so as to jet a trace gas TG into the exhaust gas G through the entrance pipe 2 and gas jet holes 7.