To downsize a bearing assembly with a rotation detection device, a magnetic generating element (2) having a directionality about a rotation axis O is provided on an inner race (21) side of the bearing assembly (20). A rotation sensor (3) capable of detecting a magnetism of the magnetic generating element (2) and outputting information descriptive of rotation or angle is fitted to an outer race (22) side in alignment with a bearing axis. The magnetic generating element (2) is fixed to a fixing member, for example, a rotary shaft (10) that is fixed to an inner diametric side of the inner race (21) of the bearing assembly (20).