To improve the initial responsiveness and the like of a motor-operated brake device by eliminating its axial backlashes with a ball screw mechanism, and reduce the size of the device by rendering its overall outside diameter small.

A caliper 3 is integrally provided on its side of an inner leg 5 with a motor case 4, in which an electric motor 9 is stored with its rotor 11 integrated with a nut cylinder 16 in a ball screw mechanism 14. The ball screw mechanism 14 is constituted of the nut cylinder 16 formed in its internal surface with a tapped hole 16A, a screw shaft 18 formed in its external surface with a spiral groove 18A, and a plurality of balls 17 interposed for rolling motion between the tapped hole 16A of the nut cylinder 16 and the spiral groove 18A of the screw shaft 18 to hold the screw shaft 18 screwed tight with the tapped hole 16A of the nut cylinder 16.