A declaration translation unit (104) compiles a declaration of variables in overlapping storage areas in a first language to a declaration of independent data variables and respective corresponding flag variables in a second language; an assignment translation unit (105) compiles an assignment to the variables in the areas in the first language so that, in the second language, whether or not the values of the variables are valid is checked assuming that the assignment to the respective overlapping data variables is performed, the results are saved in flag variables for respective variables, and the assignment is performed only to valid data variables; and a reference translation unit (106) compiles the reference of the variables in the areas in the first language so that, in the second language, if the flag variable is valid, the value of the data variable is returned, and if invalid, an exception is occurred.