Different from the usual media, the radiation field in the metamaterial behaves as the inward waves. The inward wave of a radiation field in the metamaterial has been studied on the electromagnetic principle of the far field radiation condition and the radiation power. It is shown that the inward wave is consistent with the causality. The FDTD numerical calculation method is used to simulate the wave propagation through an infinite long metamaterial cylinder, which is with a line source in its center. Therefore, the inward wave of the radiation field is validated in the metamaterial.