Assertions are used for unit testing in Object-Oriented Programming. With assertions, a test programmer compares with an expected value the result of a test execution which is obtained by referring to the object containing its result in the object being tested. Therefore, there is a problem that test classes described as testing code depend strongly on the unnecessary objects which are not of concern to unit testing. In this work we developed an assertion mechanism which traverses objects automatically in order to eliminate the dependency from/to such unnecessary objects. Furthermore, we performed experiments on open source products, and confirmed that using this assertion mechanism decreases the coupling between a test class and other classes.