A method of measuring characteristics of a critical orifice type constant flow rate instrument (CFO1, CFO2) applied to a dilution mechanism constructed in a manner that, each of a plurality of dilution units (31, 32, 33) that are connected in series is constructed such that a confluence (Lla, L2a, L3a) is provided at an intermediate portion of a main flow passage (L1, L2, L3) connecting between an input terminal (31i, 32i, 33i) and an output terminal (31o, 32o, 33o) for leading input gas; each of the dilution units (31, 32, 33) has a diluent gas flow passage (D1, D2, D3) provided with flow rate control means (MFC1, MFC2, MFC3) that is connected to the confluence (Lla, L2a, L3a), which flow rate control means (MFC1, MFC2, MFC3) are configured to introduce during operation of the dilution mechanism diluent gas of a prescribed flow rate to the main flow passage (L1, L2, L3), in order to have a predetermined dilution ratio; and at least one of the dilution units (31, 33) comprises a branch point (L1b, L3b) at the intermediate portion of the main flow passage (L1, L2, L3) so that a derivation flow passage (E1, E3) provided with the critical orifice type constant flow rate instrument (CFO1, CFO2) is connected to the branch point (L1b, L3b) so as to derive gas of a constant flow rate from the main flow passage (L1, L3) during operation of the dilution mechanism, the method being characterized in that for calibration of the critical orifice type constant flow rate instrument (CFO1, CFO2) the diluent gas from the diluent gas flow passage (D1, D3) of the one dilution unit (31, 33) comprising the branch point (L1b, L3b) and the diluent gas from the diluent gas flow passages of one or more other dilution units are introduced to the derivation flow passage (E1, E3) of the one dilution unit (31, 33) by controlling the respective flow rate control means (MFC1, MFC2, MFC3) in a manner that the total flow rate thereof is equal to the constant flow rate that is also used during operation of the dilution mechanism, and that the flow rate of the diluent gas from the one dilution unit (31, 33) comprising the branch point (L1b, L3b) is equal to the prescribed flow rate that is also used during operation of the dilution mechanism, and the flow rate characteristics of the corresponding critical orifice type constant flow rate instrument (CFO1, CFO2) are measured based on at least an upstream side pressure of the critical orifice type constant flow rate instrument (CFO1, CFO2) in the corresponding derivation flow passage (E1, E3). each of a plurality of dilution units (31, 32, 33) that are connected in series is constructed such that a confluence (Lla, L2a, L3a) is provided at an intermediate portion of a main flow passage (L1, L2, L3) connecting between an input terminal (31i, 32i, 33i) and an output terminal (31o, 32o, 33o) for leading input gas; each of the dilution units (31, 32, 33) has a diluent gas flow passage (D1, D2, D3) provided with flow rate control means (MFC1, MFC2, MFC3) that is connected to the confluence (Lla, L2a, L3a), which flow rate control means (MFC1, MFC2, MFC3) are configured to introduce during operation of the dilution mechanism diluent gas of a prescribed flow rate to the main flow passage (L1, L2, L3), in order to have a predetermined dilution ratio; and at least one of the dilution units (31, 33) comprises a branch point (L1b, L3b) at the intermediate portion of the main flow passage (L1, L2, L3) so that a derivation flow passage (E1, E3) provided with the critical orifice type constant flow rate instrument (CFO1, CFO2) is connected to the branch point (L1b, L3b) so as to derive gas of a constant flow rate from the main flow passage (L1, L3) during operation of the dilution mechanism, the method being characterized in that for calibration of the critical orifice type constant flow rate instrument (CFO1, CFO2) the diluent gas from the diluent gas flow passage (D1, D3) of the one dilution unit (31, 33) comprising the branch point (L1b, L3b) and the diluent gas from the diluent gas flow passages of one or more other dilution units are introduced to the derivation flow passage (E1, E3) of the one dilution unit (31, 33) by controlling the respective flow rate control means (MFC1, MFC2, MFC3) in a manner that the total flow rate thereof is equal to the constant flow rate that is also used during operation of the dilution mechanism, and that the flow rate of the diluent gas from the one dilution unit (31, 33) comprising the branch point (L1b, L3b) is equal to the prescribed flow rate that is also used during operation of the dilution mechanism, and the flow rate characteristics of the corresponding critical orifice type constant flow rate instrument (CFO1, CFO2) are measured based on at least an upstream side pressure of the critical orifice type constant flow rate instrument (CFO1, CFO2) in the corresponding derivation flow passage (E1, E3).