A differential amplifier circuit for use in a ring oscillator includes first and second MOS transistors to each source of which an operating power source voltage is applied, and which individually respond to first and second input signals with mutually contrary phases applied to gates thereof; cross-coupled first and second-stage transistors of which each drain-source channel is connected between each drain of the first and second MOS transistors and a ground voltage level; a first variable resistance, which is connected between a drain of the first MOS transistor cross-connected to a second gate of the cross-coupled second-stage transistors, and a first gate of the cross-coupled first-stage transistors, and which is controlled by the operating power source voltage applied to a gate thereof; and a second variable resistance, which is connected between a drain of the second MOS transistor cross-connected to a second gate of the cross-coupled first-stage transistors, and a first gate of the cross-coupled second-stage transistors, and which is controlled by the operating power source voltage applied to a gate thereof. Accordingly, the ring oscillator can ensure a greater resonance frequency range as compared with the conventional oscillator, and, at the same time, jitter characteristics are improved.