A simulation apparatus is composed of an integrated plant model process unit 1, virtual ECUs 50A to 50C that simulate other controllers, and a simulation management unit 2 that manages the execution of an input and output process to a real ECU 300, a process by the integrated plant model process unit 1, and a process by the virtual ECUs 50A to 50C, and so on, and the simulation management unit 2 causes the integrated plant model process unit 1, the virtual ECUs 50A to 50C, and so on to execute the processes required for the operation of the real ECU 300 during a given period on the basis of the operation of the real ECU 300.