PURPOSE: To accurately measure the concentration of standard gas to be supplied at the time of use by providing a branch pipe to a duct which supplies standard gas containing nitrogen dioxide, guiding part of it to a quantitative device and a integrating volume flowmeter connected thereto, and measuring the total amount of the nitrogen dioxide and the total volume of the standard gas.

CONSTITUTION: A cock 2 is opened to supply the nitrogen dioxide which has constant concentration to the duct from a standard gas supply device 1. The standard gas passes through the branch pipe 4 and is controlled by a cock 2 to a proper flow rate and guided to the nitrogen dioxide quantitative device 5, and the integrating volume flowmeter 6 measures the total amount of the passing gas. The gas passing through the duct 3, on the other hand, is guided to a measuring instrument 7 to be tested. The concentration of the nitrogen dioxide in the standard gas supplied to the measuring instrument 7 is accurately found from the amount of the nitrogen dioxide accurately determined by the quantitative device 5 and the flow rate accurately measured by the flowmeter 6.