To suppress a great change of a dilution ratio caused by fluctuation of the pressure applied to a capillary.

Indoor air is taken, and refined air refined by an adsorbent 20 and silica gel 22 which is used as zero gas is pressurized by a pump 26 and sent to a zero gas pressure governor 30. The zero gas is adjusted to have a prescribed pressure by the pressure governor 30, and adjusted to have the flow rate of about 1 L/min by zero gas capillaries formed by connecting in parallel a plurality of capillaries 34-1 to 34-6. On the other hand, standard gas filled in a container is sent to a standard gas pressure governor 8 through a solenoid valve 4, supplied similarly to a capillary 12 with a prescribed pressure, and adjusted to have the flow rate of about 10 mL/min. The refined gas and the standard gas are mixed together in a gas mixing part 16, and thereby span gas having the objective concentration is acquired and supplied from a span gas outlet 18.