To provide a PLL (FLL) circuit which uses a smaller circuit space, together with a measuring circuit having specific parameters, and can be manufactured with high cost-effectiveness.

This PLL (FLL) circuit comprises a phase (frequency) comparator 6, which controls output signals so as to make a phase (frequency) error small by comparing the phase relation (frequency) of the output signals of a follow-up oscillator 3 with that of the output signals of a reference oscillator 2, when the output signals which characterize a phase (frequency) shift are supplied to the oscillator 3; the controllable oscillator 3 in which the phase relation (frequency) of signals is affected by external parameters, the reference oscillator 2 whose signals have a constant phase relation (frequency); and an external parameter monitoring device 10 which receives signals representing the output signals, which characterize a phase (frequency) shift and converts the signals into a measured value, that represents the actual value of the external parameters.