The present invention provides a preload control device of a magnetic bearing, which includes a main shaft, a housing, a first magnetic bearing mechanism, a second magnetic bearing mechanism, a pressing mechanism, and a sliding member. The main shaft is rotatably formed in the shape of a rod that is longitudinally long. The housing covers and is fixed outside the main shaft. The first magnetic bearing mechanism is disposed between the housing and the main shaft. The second magnetic bearing mechanism is spaced from the first magnetic bearing mechanism in the axial direction of the main shaft, between the housing and the main shaft. The pressing mechanism is disposed between the second magnetic bearing mechanism and the frusto-conical member. The sliding member is disposed between the pressing mechanism and the second magnetic bearing mechanism.