1. An apparatus for realizing a runtime system for a class-based object-oriented programming language, comprising: a storage unit for storing a first class that is an existing class in the object-oriented programming language, and a second class that is a class that includes a member that is accessible from outside of the first class and that is a class which is specialized for a specific use; and a processing unit for performing processing using the second class in accordance with a predetermined instruction in software that realizes the runtime system, and performing processing using the first class in accordance with an instruction to check an identity of the second class in a user program product that is executed by the runtime system.