To fast run a compiled object program by extracting correctly the reduction, i.e., a loop pattern that frequently appears in a source program out of this source program.

The reduction is detected out of a loop (Step 21) and then converted into a loop that can be carried out effectively and in parallel (Step 22). Finally, the reduction communication is generated based on the converted loop and optimized (Step 23). The detection of reduction consists of three sub- steps. That is, a model is first produced in regard to the mask expression masking every substitute sentence in the loop. In other words, an expression tree concerning the execution condition of every substitute sentence is generated. Then it's decided whether the reduction consists of an expression tree of merged substitute sentences based on the expression tree of a prescribed conditional expression and also the data dependence relation.