PURPOSE: To constantly hold the relative speed of a recording medium and recording/reading means by providing the means to measure the period of the reading data given from a head, 1st and 2nd comparing means to output the minimum value and maximum value by comparing the measurement data and a relative displacement speed control means.

CONSTITUTION: The signal recorded on a recording medium 15 and having a preset period is read by a recording/reading means 16, the period is measured in plural times by a measuring means 23 counter and its output is compared by the 1st comparing means 12. The minimum one of the compared outputs is given to the 2nd comparing means 13. This motion is repeated several times, the given data are compared in the 2nd comparing means, the maximum value thereof is outputted and the output thereof is given to a control means 14. In this way the relative displacement speed of the recording medium and recording/reading means is constantly controlled.