When a source program written in Java or the like and using methods of BigDecimal class, a priority conforming to a predetermined arithmetic rule is identified for an arithmetic operational method included in an arithmetic operation statement, and the arithmetic operational method and the identified priority are registered into a storage device. Then, in a case where it is judged based on data stored in the storage device that a plurality of arithmetic operational methods are included in an arithmetic operational statement, and an arithmetic operational method with low priority is defined as an instance of an arithmetic operational method with high priority, following processing is carried out to generate specification data, that is, the arithmetic operational method with low priority and an instance and argument of the arithmetic operational method with low priority are converted into a first subexpression using an arithmetic operational sign in accordance with a predetermined rule. Then, parentheses representing that the arithmetic operation are carried out with priority is added to the first subexpression. In addition, the arithmetic operational method with high priority and an argument of the arithmetic operational method with high priority are converted into a second subexpression using an arithmetic operational sign in accordance with the predetermined rule. Finally, the second subexpression is linked with the first subexpression.