A method and system where a plurality of ECU is rapidly executed while avoiding deadlock by performing conservative event synchronization. The simulation system is provided with 4 layers, namely a processor emulator which is an ECU emulator, a plant simulator, a external peripheral scheduler, and a mutual peripheral scheduler. The external peripheral scheduler performs advanced execution of the plant simulator only during processor emulator reaction delay time (or the time until the next event). The notification to perform advanced execution of the plant simulator is provided until the actual plant simulator stop time. The mutual peripheral scheduler provides notification to the processor emulator to perform advanced execution only during communication delay timing between processor emulators (or the time until the next event). The processor emulator conservatively processes until the earliest time of the notification times. Each peripheral scheduler proceeds with processing until the earliest time of the accepted events.