A speaker unit having an ultra-slim structure capable of improving low-pitched sound characteristics and sound pressure, the speaker unit comprising: a frame being provided with an inner wall, the inner wall protruding toward an inside of the frame, thereby creating within the frame a space where a damper is provided, the frame further comprising an outer wall being formed outward of the inner wall, and a bottom surface below the space, wherein a yoke with a permanent magnet is accommodated within the inner wall at a center of the frame; a rim edge in the form of a stepped portion being provided at an upper end of the outer wall and an edge of a vibration plate eing fixedly bonded to the rim edge, wherein the damper includes an outer rim and an inner rim, wherein the outer rim is placed on the bottom surface and fixedly bonded thereto, or fixedly locked with a locking protrusion of a support wall constituting the outer wall while being slightly lifted upward from the bottom surface, the inner rim is located inside and above the outer rim while being spaced apart from the outer rim by a predetermined distance for enabling the inner rim being lifted upward from the bottom surface by a predetermined distance corresponding to the maximum amplitude of the up-down vibration of the vibration plate, wherein state crank-shaped elastic bars are connected to an upper end of the outer rim and a lower end of the inner rim for enabling the inner rim being vibrated up and down with the maximum amplitude of the up-down vibration of the vibration plate, the outer rim and the inner rim of the damper are made from a plate having a predetermined width and are, with respect to the bottom surface, vertically erected for vertical reinforcement, and the elastic bar is horizontally laid perpendicularly to the outer rim and the inner rim in order to improve elasticity thereof, the edge of the vibration plate is fixedly bonded to the rim edge of the outer wall and the vibration plate is fixedly bonded to an upper end of the inner rim, a voice coil is fixed to the vibration plate and accommodated in the inner rim to move up and down while being embedded between the yoke and the permanent magnet, and wherein the outer wall is constituted of a plurality of support walls and opened by openings to fix the damper and to treat the sound pressure generated upon the up-down vibration of the vibration plate. a frame being provided with an inner wall, the inner wall protruding toward an inside of the frame, thereby creating within the frame a space where a damper is provided, the frame further comprising an outer wall being formed outward of the inner wall, and a bottom surface below the space, wherein a yoke with a permanent magnet is accommodated within the inner wall at a center of the frame; a rim edge in the form of a stepped portion being provided at an upper end of the outer wall and an edge of a vibration plate eing fixedly bonded to the rim edge, wherein the damper includes an outer rim and an inner rim, wherein the outer rim is placed on the bottom surface and fixedly bonded thereto, or fixedly locked with a locking protrusion of a support wall constituting the outer wall while being slightly lifted upward from the bottom surface, the inner rim is located inside and above the outer rim while being spaced apart from the outer rim by a predetermined distance for enabling the inner rim being lifted upward from the bottom surface by a predetermined distance corresponding to the maximum amplitude of the up-down vibration of the vibration plate, wherein state crank-shaped elastic bars are connected to an upper end of the outer rim and a lower end of the inner rim for enabling the inner rim being vibrated up and down with the maximum amplitude of the up-down vibration of the vibration plate, the outer rim and the inner rim of the damper are made from a plate having a predetermined width and are, with respect to the bottom surface, vertically erected for vertical reinforcement, and the elastic bar is horizontally laid perpendicularly to the outer rim and the inner rim in order to improve elasticity thereof, the edge of the vibration plate is fixedly bonded to the rim edge of the outer wall and the vibration plate is fixedly bonded to an upper end of the inner rim, a voice coil is fixed to the vibration plate and accommodated in the inner rim to move up and down while being embedded between the yoke and the permanent magnet, and wherein the outer wall is constituted of a plurality of support walls and opened by openings to fix the damper and to treat the sound pressure generated upon the up-down vibration of the vibration plate.