Abstract: A bearing device for an axle comprising an outer ring member, an inner ring member arranged coaxial with the outer ring member and relatively rotatably through a roller, and a rotational shaft member having the inner ring member integrally connected to at least a shaft end side portion by means of a nut engaged with a shaft end portion and being secured to the axle side at a counter shaft end side portion, comprising a locking means for enlarging a first thin-wall tubular portion formed in the shaft end portion of the rotational member and a second thin-wall tubular portion formed on the nut threadedly fitted in the shaft end portion of the rotational member outwardly in a diametrical direction to deform the first and second thin-wall tubular portions outwardly in a diametrical direction to thereby prevent looseness of the nut.