PURPOSE: To secure a flow rate necessary for the expansion of a 2nd stage by connecting a sampling gas tube to the uppermost-stage pressure reduction part of a multistage expansion nozzle where a throat part is provided.

CONSTITUTION: One sampling nozzle 2' is installed in a high-pressure process line 1 and the other is connected to a 1st stage nozzle 7. Further, the nozzle 7 is connected to a 2nd stage nozzle 8 and high-pressure dilute gas flow in the nozzle 7. The pressure in the line 1 is denoted as P1 and sampling gas is put in the line 1, which is connected to the throat part 7' of the nozzle 7. Gas 6 is supplied to the nozzle 7 and then process gas is diluted and mixed while sucked by the differential pressure between the pressure at the throat part 7' and the pressure P1 in the line 1. Then, a throat part 8' is determined by a nozzle 8 in consideration of the flow rate after the dilution at the 1st stage to enable pressure-reduced expansion almost to the atmospheric pressure at the exist of the nozzle 8. The mixed gas 9 at this exit is admitted into a room-temperature dust measuring instrument thereafter and measured.