An arithmetic device simultaneously processes a plurality of threads and may continue the process by minimizing the degradation of the entire performance although a hardware error occurs. An arithmetic device 100 includes: an instruction execution circuit 101 capable of selectively executing a mode in which the instruction sequences of a plurality of threads are executed and a mode in which the instruction sequence of a single thread is executed; and a switch indication circuit 102 instructing the instruction execution circuit 101 to switch a thread mode.