A predicted outflow rate (Qo) at which gas is discharged from a process chamber 2 via a vacuum pump 3 is computed, and an input flow rate (Qi) is calculated in order to reach a preset target pressure (Psp). The input flow rate (Qi) is calculated, on the basis of the expression Qi=Qo+(ΔP/Δt)V, from a known volume (V) of the process chamber 2 and a pressure change rate (ΔP/Δt) obtained from the current pressure (P1) within the process chamber 2 to reach the target pressure (Psp). A current predicted outflow rate (Qo) is estimated on the basis of the expression Qo(n)=P2*f1(P2), from the current pressure (P2) within the vacuum pump 3 and a known characteristic pumping rate (Sp=f1(P2)) of the vacuum pump 3 under preset pressure.
