1. A cache memory unit comprising: a cache memory, placed between processors and a main memory, to temporarily store data in units of lines of given size retrieved from the main memory and store control information associated with data of the lines it retains; a group definition controller that is used to define grouping of lines to be put under management; a group selector to determine one of a plurality of groups defined by means of said group definition controller as the group to which a line to be fetched belongs, according to the information in a command that has been issued from one of the processors to fetch data from said main memory; a policy definition controller that is used to set policy of storing whereby to store lines into the same-index entries on the banks of the cache memory in one of the patterns of combinations of a plurality of groups, to any of which the lines belong, according to said grouping; a hit checker to judge whether a line specified to be read in the command from one of the processors remains stored on the cache memory; a replace line selector to determine a line to be replaced and removed from the cache memory in accordance with the group to which a new line belongs, determined by said group selector, the line control information retained in said cache memory for the candidate lines to be replaced, and the policy of storing set by means of said policy definition controller if the judgment of said hit checker is a missed hit; and a storing means to retrieve said line specified to be read from the main memory and store the line with control information for the line in place of the line determined to be replaced by said replace line selector into the cache memory if the judgment of said hit checker is a missed hit.