A vector computer system includes a vector processor configured to issue a vector store instruction which includes a plurality of store requests; a cache memory of a write back system provided between the vector processor and a main memory; and a write allocate determining section configured to generate an allocation control signal which specifies whether the cache memory operates based on a write allocate system or a non-write allocate system. When the vector processor issues the vector store instruction, the write allocate determining section generates the allocation control signal to each of the plurality of store requests based on a write pattern as a pattern of target addresses of the plurality of store requests. The cache memory executes each store request based on one of the write allocate system and the non-write allocate system which is specified based on the allocation control signal.