A failure detection device that detects a failure occurring in an evaporative fuel purge system by detecting the pressure inside a vapor passage and a purge passage. The vapor passage and the purge passage are connected via a bypass passage. A pressure sensor is connected to either the vapor passage or the purge passage. An engine control unit is provided which judges that a failure has occurred in the evaporative fuel purge system when the pressure sensor detects a pressure equal to or higher than the predetermined pressure.