Provided is a PLL apparatus realizing extremely high frequency stability. As a concrete means for solving the problem, an A/D (analog/digital) conversion unit samples a standard signal based on 40 MHz frequency signal, which is a rectangular wave, from an oven controlled crystal oscillator (OCXO), and an orthogonal transformation unit applies orthogonal transformation to a digital signal from the A/D conversion unit to extract a real part (I) and an imaginary part (Q) which are complex expression of a phase vector equivalent to a phase difference between the standard signal and the frequency signal from the OCXO. An angular velocity of this vector is detected and a direct-current voltage according to the angular velocity is generated and supplied to the OCXO via a PWM control unit.