There are provided a failure determination device simple in construction and capable of properly determining failure of a humidity sensor that detects the humidity of exhaust gases from an internal combustion engine and a control system for an exhaust passage changeover valve, which can switch the changeover valve with appropriate timing, thereby enabling sufficient purification of exhaust gases. In one form of the invention, an operating condition of the engine is detected. It is judged based on the detected operating condition of the engine whether or not the engine is in an operating condition in which failure determination of the humidity sensor can be executed. It is determined whether or not the humidity sensor has failed, based on a result of detection by the humidity sensor, when it has been judged that the failure determination of the humidity sensor can be executed. In another form of the invention, a changeover valve switches an exhaust passage having a catalytic device arranged therein for purifying exhaust gases from the engine, between a main exhaust passage and a bypass exhaust passage bypassing the main exhaust passage and having an adsorber arranged therein which is capable of adsorbing hydrocarbons and water in the exhaust gases. A humidity sensor is arranged at a location downstream of the adsorber in the bypass exhaust passage, for detecting humidity of the exhaust gases guided into the bypass exhaust passage. The changeover valve is driven based on a result of detection by the humidity sensor.