Deterioration in operation performance due to a fault diagnosis is prevented. A semiconductor device 90 according to the present invention includes a plurality of CPU cores 91 to 94 each including a scan chain, and a diagnostic test circuit 95 that performs a scan test for the plurality of CPU cores 91 to 94 by using the scan chain of the CPU core. The diagnostic test circuit 95 performs a scan test for each of the plurality of CPU cores 91 to 94 in a predetermined order on a periodic basis so that execution time periods of the scan tests do not overlap each other.