An optimum control parameter in control of an internal combustion engine and the like is searched. In a plurality of search cycles, a control parameter that maximizes an output of an object to be controlled which shows an output realized by a given control parameter is searched using control parameters. The control parameters are provided at each search cycle by a predetermined algorithm. A periodic function of a predetermined period and a correction value obtained in a previous search cycle are added to the control parameters to obtain an input parameters to the object. An output obtained from the object with the input parameters is multiplied by the periodic function to obtain a correction value for correcting the control parameters such that the search converges.