In this paper, four simple nonlinear circuits with time-varying resistors are analyzed. These circuits consist of only four elements; a inductor, a capacitor, a diode and a time-varying resistor and are a kind of parametric excitation circuits whose dissipation factors vary with time. In order to analyze chaotic phenomena observed from these circuits a degeneration technique is used, that is, diodes in the circuits are assumed to operate as ideal switches. Thereby the Poincar maps are derived as one-dimensional maps and chaotic phenomena are well explained. Moreover, validity of the analyzing method is confirmed theoretically and experimentally.