PURPOSE: To make it possible to perform accurate dilution, by providing a rotor, which is slidably contacted with a sheet, which has a diluting gas introducing port, an introducing port for gas to be diluted, and outlet ports, in a recess part of a circular cylinder housing, so that the rotor is freely rotated; and providing communicating paths, through which gas is made to flow to the outlet ports so as to face the introducing ports selectively, in the rotor.

CONSTITUTION: In a recess part 2 of a circular cylinder housing 1, a rotor 3 is rotatably accommodated. In a Teflon sheet 5, which is slidably contacted with the lower surface of a rotor 3, a diluting gas introducing port 7, an introducing port 8 for gas to be diluted, an outlet port 9, through which mixture of the gas to be diluted that is mixed with and the diluting gas is exhausted, and an outlet port 10, through which mixture of the diluting gas and that is mixed with the gas to be diluted a is exhausted, are provided in an ark shape at an equal interval. In the Teflon sheet 5, which is slidably contacted with the rotor 3, a communicating path 11 facing the introducing port 8 and the outlet port 9 and a communicating path 12 facing the introducing port 7 and the outlet port 10 are provided in an ark shape at an equal interval. A plurality of through holes 13 are provided in the rotor 3 as communicating paths to the outlet ports 9 and 10. Thus accurate dilution is performed without the effect of viscosity, which is varied by the kinds of gases.