Three-dimensional magnetohydrodynamic (MHD) simulations are carried out in order to reveal the fundamental mechanism of the self-sustainment process in the reversed field pinch (RFP) plasma. It is confirmed that the RFP configuration is sustained in a cyclic process, where the MHD relaxation phase and the resistive diffusion phase appear cyclically and alternatively. In the MHD relaxation process, the RFP plasma approaches a Taylor's minimum energy state, but departs from there in the diffusion process. In other words, since MHD relaxation processes periodically release excess magnetic energy accumulated in the resistive diffusion phase, the RFP plasma can stay in the neighbourhood of the minimum energy state. The mechanism of this cyclic process is disclosed. Specifically, when at least two ideal kink (m = 1) modes become unstable, MHD relaxation can take place. This is because the MHD relaxation progresses through non-linear reconnection of the m = 0 mode, which is driven by non-linear coupling between the unstable kink modes. Therefore, self-sustainment processes can be achieved by the non-linear effects of, essentially, m = 0 and m = 1 modes. The quantitative dependence of the relaxation-diffusion cycle on the aspect ratio of the device is considered, along with its dependence on the magnetic Reynolds number. These results are consistent with recent experiments and indicate that a coherent oscillation, which is often observed in experiments, is necessary for self-sustainment. The influence of self-sustainment processes on particle confinement is briefly discussed.