The relativistic covariance of the S-matrix is studied for a charged vector meson with an arbitrary value of magnetic moemnt interacting with the electromagnetic field. It is explicitly shown that within the framework of the ordinary quantum electrodynamics the covariant result can be obtained for the self-energy of a vector meson, if we treat the divergent integral properly. A more general and simpler method is also presented to make the whole process of the calculation of the S-matrix completely covariant. The application of the similar method to other types of interaction is also discussed.