In a semiconductor device including a lockstep function, conflicts of bus accesses by a plurality of processors are suppressed. The semiconductor device includes a first processor, a second processor for monitoring operation of the first processor in a first mode, first and second buses, first and second non-shared resources dedicated to either the first or second processor in a second mode, and a first selector for selecting a bus for transferring interface signals between the second processor and the selected bus. In a second mode in which the first and second processors execute different instructions, the first selector selects the second bus. In the second mode, the first non-shared resource is accessed by the first processor via the first bus and the second non-shared resource is accessed by the second processor via the second bus.