To provide a phase-locked oscillator and a communication device which are capable of coping flexibly with variations in an environmental condition and being maintained stably high in service quality, where the communication device is capable accessing a desired transmission line, by making signals generated by the oscillator which undergo the prescribed processing.

A phase-locked oscillator is equipped with an oscillation means, which is provided in a phase-locked loop, contains a reactance elements, and generates signals of prescribed frequency; and a limit discriminating means, which monitors the state of the phase-locked loop and changes the reactor in reactance, when it discriminates at the time prior to a limiting point, at which the phase-locked loop is capable of maintaining its phase-locked state.