A centrifuge rotor cover has a lid, a circular plate attached to the lid with click holes formed in a circle, a knob and knob shaft, a plate spring attached to a surface of the knob facing the circular plate and an elastic body pushing the lid against the rotor depending on the distance between the knob and the lid. If this distance is equal to or less than a predetermined value, the plate spring is caught repeatably in the click holes as the knob is turned to cause sounds and vibrations that indicate the knob is not tightened securely.