1. A program conversion method comprising: converting a program by controlling a computer when an execution object of said program transitions between two code columns in different environments, including: a first step of setting a portion of code not to be considered in program transformation between codes before transition, said portion of code affecting the program transformation on a transitioned code column and storing the portion of code in memory, and a second step of performing said program transformation for said transitioned code column without considering said portion of code on the basis of said information stored in said memory.