PURPOSE: To improve working efficiency by providing a static pressure measuring hole and a thermometer on a measuring tube connected to an exit of an air blower and five or more kinds of interchangeable apertures wherein a relation of an air amount to the static pressure at the tip of the tube has been measured.

CONSTITUTION: An appropriate one of apertures 6 is selected to measure a relation in static pressures at a hole 4 for measuring an air amount vs. static pressure on a measuring tube 3 in advance, and a characteristic curve of the static pressure vs. air amount in a standard air state is created. Then static pressure is measured by the static pressure measuring hole 4 and also air temperature in the tube 3 is measured by a thermometer 5, and this is converted into static pressure in the standard air state, with which an air amount is calculated from the characteristic curve. Thus measurement of static pressure of an air blower can be done at the same time as measurement of static pressure at the static pressure measuring hole 4, so that working efficiency can be improved.