The formula of the Molien function in the case of corepresentation is derived by application of the symmetry theory for second order phase transition to magnetic systems, it is different from that of unitary groups. The analysis of the magnetic phase transition of FeRh alloy shows that eqivalent representation in the sense of corepresentation may lead to different symmetry breaking for the reason of none-quivalent characters. The result that a continuous magnetic phase transition Oh1⊕θOh1→C4v1⊕θD4h1-C4v1 exists theoretically is supported by experiments.