A clock supply device includes a receiving unit configured to receive frame synchronization packets from an asynchronous network and generate timing signals; a phase comparing unit configured to perform phase comparison by comparing phases of the timing signals generated by the receiving unit and clock signals generated by an internal oscillating unit; a phase variation detection unit configured to detect a frequency variation of the frame synchronization packets based on a trend of a variation amount of a phase difference that is obtained by performing a statistical process on count values obtained as a result of the phase comparison; and an oscillating frequency control unit configured to control an oscillating frequency of the internal oscillating unit when the phase variation detection unit detects the frequency variation of the frame synchronization packets.