PROBLEM TO BE SOLVED: To provide a sewage treatment process simulation method for automatically setting parameter, reducing an estimate error with good sensitivity and achieving the shortening of time, and an apparatus therefor. SOLUTION: Advancing biochemical reaction is simulated by inputting the data such as water quality, flow rate, temperature or the like of a sewage treatment plant and the parameter and state variable constituting a biochemical reaction model. The actually measured value of water quality and a calculation result are compared and, until the difference between them becomes sufficiently small, the parameter is altered to repeat simulation to optimize a parameter value and final simulation operation is performed on the basis of the parameter value. In this method, the sensitivity of the parameter is analyzed in order to preferentially alter the parameter large in the effect exerted on water quality and the parameter altered preferentially is selected by evaluating the analyzed result of of sensitivity and simulation is repeated by altering the selected parameter value to optimize the parameter value. COPYRIGHT: (C)2004,JPO