PURPOSE: To avoid collision, to minimize the memory capacity to be assigned, and to optimize hashing by providing a hash circuit, a hash tag memory, a key code memory, etc.

CONSTITUTION: A new data word from a register 32 and a prefix from a prefix register 33 are supplied to a hash circuit 34 to generate a hash address and an abbreviated key. The hash address is supplied to hash tag memories 20 and 21; and if a tag bit value corresponding to the hash function is one, contents of the memory 21 are used as an address in the next cycle to read and compare the abbreviated key. If the ab-breviated key doesn't match as the result and the tag bit in a key code word memory 22 is 0, a failure following collision is decided to invalidate the result of hash operation, and preceding data in a data register 41 is used as a new correct prefix. Thus, collision is avoided, and the memory capacity is minimized, and hashing is optimized.