The present specification discloses a diagnosis system and an optimum control unit for an internal combustion engine. The basic concept of the present invention resides in that a random retrieved signal of which auto correlation function is an impulse shape is superposed on a signal of an internal combustion engine, said superposed signal is used to measure a change of an operation state of the internal combustion engine, and an optimum direction of a control value is detected by a correlation between said measured value and retrieved signal. This method includes the steps of superposing a search signal for fine adjusting a fuel flow quantity value and an ignition timing on a fuel flow quantity signal and an ignition timing signal respectively, applying the fuel flow quantity signal and the ignition timing signal superposed with said search signal respectively to the internal combustion engine, detecting a value of a parameter showing a revolution number or an operation state of the internal combustion engine in response to the superposed signals, detecting a correlation between the detected value and the search signal, and carrying out diagnosis or control of the internal combustion engine based on the detected correlation.