A seat lifter device comprising: a seat support member; a front link including first and second ends, the first end being rotationally coupled to a front section of the seat support member; a rear link including first and second ends, the first end being rotationally coupled to a rear section of the seat support member; a frame member including a front section rotationally coupled to the second end of the front link and a rear section rotationally coupled to the second end of the rear link at a position rearward of the coupling portion between the seat support member and the rear link, wherein a seat back is supported at a rear end portion of the frame member; a cushion member (24, 45) including part of a seat cushion, the cushion member (24, 45) including a front section rotationally coupled to the frame member and a rear section rotationally coupled to the rear link at a position further separated from the coupling portion between the seat support member and the rear link than the coupling portion between the rear link and the frame member; and a rotation transmitting member for rotating one of the front link and the rear link, the seat lifter device being characterized in that the rear link is one of a pair of rear links facing each other in a seat widthwise direction, the seat lifter device further comprises a beam member, the beam member including a pair of shaft portions (41 a), which are secured to the rear links and provide the coupling portions between the rear links and the frame member, and an eccentric portion (41 b), which has an axis displaced from the axis of the shaft portions (41 a) and extends between the shaft portions (41 a), and the rear section of the cushion member (24, 45) is rotationally coupled to the rear links via the eccentric portion (41 b). a seat support member; a front link including first and second ends, the first end being rotationally coupled to a front section of the seat support member; a rear link including first and second ends, the first end being rotationally coupled to a rear section of the seat support member; a frame member including a front section rotationally coupled to the second end of the front link and a rear section rotationally coupled to the second end of the rear link at a position rearward of the coupling portion between the seat support member and the rear link, wherein a seat back is supported at a rear end portion of the frame member; a cushion member (24, 45) including part of a seat cushion, the cushion member (24, 45) including a front section rotationally coupled to the frame member and a rear section rotationally coupled to the rear link at a position further separated from the coupling portion between the seat support member and the rear link than the coupling portion between the rear link and the frame member; and a rotation transmitting member for rotating one of the front link and the rear link, the seat lifter device being characterized in that the rear link is one of a pair of rear links facing each other in a seat widthwise direction, the seat lifter device further comprises a beam member, the beam member including a pair of shaft portions (41 a), which are secured to the rear links and provide the coupling portions between the rear links and the frame member, and an eccentric portion (41 b), which has an axis displaced from the axis of the shaft portions (41 a) and extends between the shaft portions (41 a), and the rear section of the cushion member (24, 45) is rotationally coupled to the rear links via the eccentric portion (41 b).