To automatically estimate an optimal value even in a control system where it is difficult to determine a target numerical value in advance.

In the adaptation system for optimally adapting the control parameter, a control part 40 generates a value for designating a target air fuel ratio in air fuel ratio control by an ECU 10 on the basis of an output of an exhaust gas analyzer 20 for analyzing each ingredient amount of exhaust gas exhausted from an engine 1, and a state evaluating part 30 evaluates a control state of the air fuel ratio on the basis of outputs of the exhaust gas analyzer 20 and an O2 sensor of the engine 1. A proper value estimating part 50 adjusts the designated value generated by the control part 40 on the basis of an evaluation result of the state evaluating part 30 to be optimally adapted to the target air fuel ratio, and finally determines a target air fuel ratio to be stored in the ECU 10. Thus, when previously storing a target air fuel ratio value in a map, it is possible to automatically and efficiently perform map preparation that requires a developer's experience. Manhour required in developing a vehicle is reduced to achieve cost reduction.