There is provided a failure diagnostic system which includes an intake channel and an exhaust channel in communication with cylinders of an internal combustion engine; the exhaust pressure increasing device that increases the exhaust system pressure of the exhaust channel; an intake system pressure detecting device provided in the intake channel, for detecting the intake system pressure; and an exhaust pressure increase failure diagnostic section that determines whether the exhaust pressure increasing device has failed or not according to intake system pressure information obtained by the intake system pressure detecting device and a predetermined failure diagnosis reference range.