To provide a bearing device for obtaining highly accurate roller rotating accuracy.

This bearing device has: a bearing 2 having a moving body between an outer race and an inner race; a housing 4 for housing the bearing 2; front lids 5 and 6 for fixing the outer race of the bearing from the side; a bearing nut 7 for fixing the bearing; and a spacer 3 arranging a distance between the bearing, or a spacer arranged between a the bearing 2 and the bearing nut 7, and supports a shaft part of a main roller in a converting device. Since the device is characterized by fixing the bearing by adjusting a fastening condition of the bearing nut 7 while measuring stress by a strain sensor by installing the strain sensor in the spacer 3 or the spacer, the bearing nut 7 is surely and uniformly fasten by controlling a preload by a numeric value. Thus, the bearing is easily adjusted in a proper installing attitude, and the highly accurate roller rotating accuracy is obtained. Time for installation and adjustment is also shortened.