A multiprocessor system can reduce a broadcast for cache memory consistency control with memory access from an I/O device. The multiprocessor system is provided with a cache memory identifier or an owner tag, and a block length table for recording a memory write block length of the I/O device. The cache memory identifier records that the cache has an exclusive copy. The owner tag records that there is no cache memory having an exclusive copy. If there is an exclusive copy during read through the I/O device, a read request is issued to both a cache holding the copy and a memory. If it is recorded that the copy is not present, data are directly read from the memory. Moreover, when a write block length is recorded in the block length table during write, whole blocks are collected to issue a request for invalidation from the cache and the request is directly written to the memory after the invalidation is completed.