To provide a preload measuring method of a bearing for measuring a preload applied to a bearing with superior accuracy by a simple method.

The preload measuring method of a bearing is a method for measuring a preload to be applied to each of bearings 6 and 8, in a transfer unit 1 which applies a preload in an axial direction to the each of the bearings 6 and 8, by injecting a right-side shim 12, in a radial direction relative to a mounting case 9, into a gap formed between a bearing holding part 7 and the right-side bearing 8, in an axial direction thereof, in a state where a ring gear 3 is engaged with a drive pinion 4. The method includes the step for measuring thrust f needed to inject the right-side shim 12 in a radial direction relative to the mount case 9 into a gap formed between the bearing holding part 7 and the right-side bearing 8, in an axial direction by using a thrust measuring device 22, and the step for calculating an axial force Fs (preload) based on the thrust f measured with the thrust measuring device 22 by using an axial force calculating device 23.