1. A semiconductor integrated circuit device which executes a plurality of threads each as a software execution unit while switching the threads synchronously with clocks, comprising: a first register set used for execution of a first thread in the threads; a second register set used for execution of a second thread different from the first thread; and a comparison circuit for comparing a result of execution of the first thread and a result of execution of the second thread when the first and second threads are execution units of the same specific software and when a redundant thread signal indicates that a redundant thread is being executed in the first and second register sets, wherein the first and second register sets are different register sets; and wherein remaining register sets of said semiconductor integrated circuit have no active redundant thread signal, thereby comprising execution units of unique software and thereby serving to execute non-redundant threads for which there are no result comparisons between threads.