In a previous report, the overall properties of self-synchronized phenomena generated in rotor-type oscillators were experimentally and analytically clarified using two types of systems constructed from oscillators and coupled mass-blocks. The relationship between the stable self-synchronized solutions and the linear natural frequencies of a spring-mass system for each system was also examined. In an effort to clarify the mechanism behind the occurrences of the self-synchronized phenomena, an investigation that is based on the nonlinear vibration characteristics of the systems should be conducted. Nonlinear normal modes have the potential to be useful tools for such an investigation, because the nonlinear normal modes and the self-synchronized phenomena are both periodic motions in nonlinear systems with many degrees of freedom. However, there is a very important difference: the former are free motions in conservative systems and the latter are self-excited motions in nonconservative systems. Thus, a definite relationship between the two is not obvious. This report examines this relationship using the same systems employed in the previous report. The computational results demonstrate that many characteristics of the self-synchronized phenomena can be explained by the nonlinear normal modes.