To exactly diagnose the abnormality of a cross bearing which is used for the spindle of a universal joint applied to a rolling mill online.

Lubricating oil is sampled at oil supplying timing when the lubricating oil for the cross bearing is aupplied, the number X5 of flat-board- shaped worn particles of 5-10 μm length and number X10 of the flat-board-shaped worn particles of 10-20 μm length are measured using an analitical ferrography method and, by executing calculation based on a discriminant on the basis of these results, the presence or absence of the generation of peeling on a cross pin is diagnosed from the result of the calculation and, by calculating the based on a multiple regression formula, the extent of damage which shows the rate of a damaged area in the area to be loaded of the cross pin is determined.