Based on a model for determining optical emission intensity values Y at wavelengths from actuator values X of an etching apparatus, X is calculated from Y to achieve preferable Run-to-Run control over Y. A relation between X and Y is defined as a control model (matrix model C1, ratio-constraint model C2) based on an algebraical expression with ΔX as an input and ΔY as an output. In etching process control, ΔX (manipulated volume) is calculated from ΔY (controlled volume) using the control model, based on a non-control optical emission intensity value Y, to set X (S1) for the etching process, during which Y is monitored. Based on an actual value for Y, a non-control optical emission intensity value Y to be used in the next wafer etching process is calculated.