In a centrifugal separator of the type in which driving power of a motor is transmitted to a rotor via a power transmission mechanism, such as a belt, a motor-rotation signal frequency fm and a rotor-rotation signal frequency fr are computed, on the basis of which a frequency ratio A (fr/fm) is computed. When the frequency ratio A exceeds the upper limit of a first predetermined range, a warning message is displayed to prompt the user to perform maintenance. When the frequency ratio A exceeds the upper limit of a second predetermined range, an alarm message is displayed and the motor is stopped.