A knob structure includes a holder, a steel ball, an elastic metallic piece, a swiveling element and a connecting element. The holder has at least one through hole. The steel ball is slidably placed inside the through hole. The elastic metallic piece and the swiveling element reside at either sides of the holder. The elastic metallic piece slidably lies against the steel ball. The swiveling element has a sliding portion which has a plurality of notches spaced at intervals. The sliding portion slidably lies against the steel ball so that the steel ball slides into or out of the notches by turning the swiveling element. The steel ball has a smooth surface and is in point-to-point contact with the sliding portion and with the elastic metallic piece. Therefore, frictions therebetween are so small that no scraps generate due to the frictional operation.