A lifter mechanism with a cushion frame unit, the lifter mechanism supporting the cushion frame unit for supporting a cushion member for a seat cushion part in a manner capable of lifting or lowering by driving a lifting driving part with respect to a left slider and a right slider constituting a seat slide device, wherein: a front link and a rear link are provided between each of side frames (11, 11) disposed on a left side and a right side of the cushion frame unit and one of the sliders, wherein pivotal, support points are provided between the front links (151, 151) and the rear links (152, 152) and the side frames (11, 11) and wherein the side frames (11, 11) have outer plate-shaped frames (111, 111) and inner plate-shaped frames (112, 112); the lifting driving part comprises a drive gear provided on one side frame out of the side frames (11, 11) and disposed between the front link and the rear link of a side of the one side frame, so as to transmit a driving force for lifting or lowering; one end of a coupling driving rod rotating with the drive gear is coupled to a rotation center of the drive gear; the lifter mechanism comprises one side connector on the side of the one side frame where the lifting driving part is provided, the one side connector extending forward from the rear link of the side of the one side frame and bridged to the drive gear, wherein the one side connector, the rear link, and the drive gear are pivotally supported with each other; the lifter mechanism comprises another side connector on a side of the other side frame where the lifting driving part is not provided to extend forward from the rear link of the side of the other side frame, and a driving link with one end coupled to a front end of the other side connector, wherein the other side connector, the rear link, and the driving link are pivotally supported with each other; and another end of the coupling driving rod with one end coupled to the rotation center of the drive gear is structured to be coupled to another end of the driving link, so as to transmit a rotation force of the drive gear to the driving link via the coupling driving rod, characterized in that a belt anchor point, which is an end portion of a beam bridged across opposing rear portions of the outer-plate shaped frames, is provided in a rear portion of the outer plate-shaped frames of the side frames of the cushion frame unit supported by the front links (151, 151) and the rear links (152, 152); and in that a portion immediately above the belt anchor point is formed by a plane inside of a bent peripheral portion of the outer plate-shaped frame so as to set a crushable zone which deforms relatively easier than a surrounding portion when an external input of a predetermined value or higher is received is set in a portion above the belt anchor point. a front link and a rear link are provided between each of side frames (11, 11) disposed on a left side and a right side of the cushion frame unit and one of the sliders, wherein pivotal, support points are provided between the front links (151, 151) and the rear links (152, 152) and the side frames (11, 11) and wherein the side frames (11, 11) have outer plate-shaped frames (111, 111) and inner plate-shaped frames (112, 112); the lifting driving part comprises a drive gear provided on one side frame out of the side frames (11, 11) and disposed between the front link and the rear link of a side of the one side frame, so as to transmit a driving force for lifting or lowering; one end of a coupling driving rod rotating with the drive gear is coupled to a rotation center of the drive gear; the lifter mechanism comprises one side connector on the side of the one side frame where the lifting driving part is provided, the one side connector extending forward from the rear link of the side of the one side frame and bridged to the drive gear, wherein the one side connector, the rear link, and the drive gear are pivotally supported with each other; the lifter mechanism comprises another side connector on a side of the other side frame where the lifting driving part is not provided to extend forward from the rear link of the side of the other side frame, and a driving link with one end coupled to a front end of the other side connector, wherein the other side connector, the rear link, and the driving link are pivotally supported with each other; and another end of the coupling driving rod with one end coupled to the rotation center of the drive gear is structured to be coupled to another end of the driving link, so as to transmit a rotation force of the drive gear to the driving link via the coupling driving rod, characterized in that a belt anchor point, which is an end portion of a beam bridged across opposing rear portions of the outer-plate shaped frames, is provided in a rear portion of the outer plate-shaped frames of the side frames of the cushion frame unit supported by the front links (151, 151) and the rear links (152, 152); and in that a portion immediately above the belt anchor point is formed by a plane inside of a bent peripheral portion of the outer plate-shaped frame so as to set a crushable zone which deforms relatively easier than a surrounding portion when an external input of a predetermined value or higher is received is set in a portion above the belt anchor point.