1. An instruction prefetching apparatus for prefetching instruction from a storage unit in a computer executing the instruction, said apparatus comprising: a branch judging means for judging whether a prefetched instruction is a branch instruction which instructs branching or not and outputting a branch signal when that instruction is a branch instruction; a determined condition information generating means for judging whether a condition deciding to perform the branching of the branch instruction is already determined or not when the branch signal is output, and outputting, to a prefetch address generating means, determined condition information corresponding to the condition judged to be determined; and a prefetch address generating means for generating an address of a prefetch target based on the determined condition information input from the determined condition information generating means.