This article describes an axial transient vibration and axial excitation response of a polygonal mirror scanner rotor driven by a flat-type brushless dc motor and supported by a passive thrust magnetic bearing and a radial air bearing. From results of experimental investigations, the following characteristics of the vibration have been deduced: (1) Repulsive magnetic force was induced between the rotor magnet and stator coils of the dc motor under current passage. The force caused axial static displacement and axial transient vibration. (2) Because the stiffness of the thrust magnetic bearing was small, large axial vibration took place due to the axial excitation. (3) Two types of damper were introduced to suppress the axial transient vibration and the axial excitation response; one was an eddy-current damper and the other was an air damper. It was demonstrated that the air damper was effective for reducing the axial vibration.