Existing semiconductor devices cannot detect a failure occurring in a circuit required for mode switching processing for other than arithmetic cores, so that reliability is inadequate. A semiconductor device of an embodiment of the invention includes: a selector which is provided corresponding to among plural arithmetic cores one used as a checking arithmetic core in lock-step mode and which, in lock-step mode, blocks the interface signals outputted from the corresponding arithmetic core and, in split mode, lets the interface signals outputted from the corresponding arithmetic core through; an access monitor which monitors the interface signals outputted via a selector and, when an abnormal state of the interface signals is detected, outputs an error signal; and an error control unit which outputs, based on the error signal outputted from the access monitor, an abnormal state processing request to a higher-order system.