A flow rate of a gas is determined based on a predetermined relational expression including, as parameters: the flow rate of the gas; diameter and length of a hole; upstream and downstream pressures; and temperature, molecular weight, viscosity coefficient, and specific heat ratio of the gas. Additionally, setting conditions for a type and temperature of the gas, the length of the hole, and the pressures upstream and downstream from the hole are set; the relational expression is used to obtain the correspondence relationship between the diameter of the hole and the flow rate of the gas flowing through the hole; an approximation function approximating the obtained correspondence relationship is determined; the flow rate of a gas passing through a test piece having a hole of an unknown diameter is measured; and the diameter of the hole is estimated, based on the measured flow rate and the approximation function.