Compatibility of a callable unit program configuring a user program is provided. A code generator that generates a code having a format executable by a control device from the user program generates an instruction code creating a structure storing association information associating a name of an argument with a value set to the argument with respect to a call expression, and converts the call expression into an instruction code calling the unit program using an identifier of the structure. A value of association information is set to the argument when the association information corresponding to the name of the argument is stored in the structure with respect to each argument of the unit program, an instruction code setting a predetermined value to the argument is generated when the association information is not stored.
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