The present invention relates to an air foil journal bearing comprising: a bump foil which has a plurality of elastic bumps, and is provided on the inside of a bearing housing and coupled thereto; a mid foil which is provided on the inside of the bump foil, is formed along the circumferential direction, and has at least one end thereof in the circumferential direction coupled to the bearing housing; and a top foil which is provided on the inside of the mid foil, is formed along the circumferential direction, and has at least one end thereof in the circumferential direction coupled to the bearing housing, wherein friction and vibration between a rotor and the top foil can be reduced, as the rotor can float at a relatively low rotation speed by additional pressure being allowed to be formed in a section in which maximum positive pressure is applied during low-speed rotation of the rotor, as a small step part is formed with the radius of the inner circumferential surface of a partial area, in the circumferential direction, of the mid foil being smaller than the radius of the inner circumferential surface of the remaining area.