Nonconforming domain decomposition methods have been considered on many works. But non-conforming case, in particular, mortar method has difficulty that the communication in the interface between subdomains is more complex than conforming case. This difficulty is an obstacle not only to numerical anaysis but also to practical implementation. Here, we made an effort to find a new test function space which is independent of discretization of subdomains. We chose polynomial space as this test function space and implemeted an iterative method.