To shorten the time needed for task switching by selecting one of cache memory banks as a selected cache memory bank associatively with a choice of a selected register bank.

A bank selecting register 7 and a selector 8 constitute a register bank selecting means A and a cache memory 9 refers to a memory word only from one cache bank indicated by the bank selecting register 7. Further, each cache bank of a cache memory 9 is composed of a tag memory and a data memory. When a task is switched, the task number of a switching destination is set in the bank selecting register 7 and then the task is switched to register banks 3 to 6 having the same number with the task number and a cache bank according to the decoding result of the value of the bank selecting register 7.