PURPOSE: To enable to perform a safe collection of gas, by a method wherein a conveyance pipe is placed between a sampling gas collecting spot in a nuclear reactor building and a building where a sampled gas is analyzed, and a sampling gas capsule is mounted at the inside of the pipe.

CONSTITUTION: A line 4 for selecting a sampling gas is coupled to a primary housing vessel 2. Collecting pipes 17 and 18 are connected in the middle of the sampling line 4 through switch valves 15 and 16, and are coupled to a conveyance pipe 19. An air pipe 21 is connected to the conveyance pipe 19, a partition valve 24 is mounted in the middle, and a sample capsule is slidably housed. In case a sample gas analysis is needed resulting from the occurrence of a trouble, a capsule 25 is attached to the conveyance pipe and the partition valve 24 is opened, and this permits a sample to be safely collected at a location away from a trouble spot.