To achieve a structure capable of estimating a preload applied to respective rolling elements 7 and 7, without determining an axial load acting on a double-row rolling bearing unit 1 for supporting a wheel.

This preload estimating device for a double-row rolling bearing unit includes an inter-row radial displacement measuring means for measuring inter-row vertical displacement being inter-row radial displacement between an outer ring 3 and a hub 4, and a preload estimating means for estimating the preload based on the inter-row vertical displacement measured by the inter-row radial displacement measuring means. The preload estimating means is constituted for example, so as to estimate the preload based on a change in a moving average value of a minimum value of the inter-row vertical displacement.