PURPOSE: To quickly perform engine control tests by using an appropriate control constant even on various kinds of engines by updating the preceding value to a new control constant which is obtained by adding an initial control-constant value to a control-constant correcting value.

CONSTITUTION: The control-constant proportional gain and each initial value KP0 and T0 of the integration term of a PI control circuit 3 are set 13 and 14 beforehand. On the other hand, an intake air (fuel) flow rate changing means 9 (10) takes in the current intake air (fuel) flow rate from an intake air (fuel) detection circuit 5 (6) and sends an intake air (fuel) flow rate variable to a proportional gain (integration term) calculating means 15 (16) after replacing the preceding value with the fetched flow rate and tracking the flow rate. Upon receiving the variable, the means 15 (16) calculates a proportional gain (integration term) correcting value KP1 (T1) from a specific expression by using the current variable and preceding value and finds a new proportional gain (integration term) KP (T) by adding the value KP1 (T1) to the initial value KP0 (T0). The found value is used for updating the gain KP and integration term T of the circuit 3 through a proportional gain (integration term) updating means 17 (18).