PURPOSE: To obtain a diluting device which is easy to operate and has good work efficiency by performing automatically operations of gas sampling, dilution, measurement, etc.

CONSTITUTION: A high concentration gas flowed in from a high concentration gas inlet 12 is sampled by an automatic gas ampler 1-1, and this sampled high concentration gas is flowed into a diluting vessel 2 and is diluted. The diluted gas is sampled by an automatic gas sampler 1-2 again. A low concentration gas inlet 13 is provided, and the low concentration gas flowed in from this inlet 13 is sampled by an automatic gas sampler 1-3. A gaseous nitrogen inlet 18 is connected to the entrance (c) of the gas sampler 1-2 through a flow passage 73. An exit (f) of the gas sampler 1-3 are connected by a flow passage 74, and the exit (f) of the gas sampler 1-3 is connected to a gas chromatograph of gas analysis through a flow passage 75 and an exit 19. Thus, all operations of gas sampling dilution, etc. are performed automatically to eliminate misoperations also to improve the work efficiency.