PURPOSE: To perform measurement with high accuracy, by providing a third bearing from a base side at a position where a spindle can stand autonomously at the time of removing the bearing at a cover side, in the spindle supporting mechanism of both sides supporting type.
CONSTITUTION: When a cover 4 is removed from a base 2, and the bearing 10 and a bearing holder 32 are removed from the spindle 6, the bearing 6 is lowered downward by being pressed by a pre-load spring 12, but a step part 26 formed in the middle of the bearing 6 is engaged with the inner ring of a bearing 24. In other words, in this state, the lower end of the spindle 6 is supported with a bearing 8, on the other hand, the upper part is supported with the bearing 24. Therefore, it is possible to make the spindle 6 stand autonomously even in a state where the cover 4 is removed. In this way, it is possible to perform the measurement of the reference plane size of the spindle 6 with high accuracy, and to improve operability at the time of assembling a disk, and furthermore, to prevent head crash due to the slidable contact of a head and the disk at the time of inserting a carriage from occurring.