A device with models constructed based on thermodynamics laws and fluid dynamics laws including the energy conservation law, momentum conservation law and mass conservation law. Compressor outflow flow rate calculation section calculates the flow rate of air that flows out of a compressor based on a relationship between an in-cylinder intake air flow rate during steady-state operation in an internal combustion engine system and supercharging pressure, which is pressure of air compressed by the compressor, and a value of the in-cylinder intake air flow rate calculated by in-cylinder intake air flow rate calculation section.