Abstract: Disclosed here is a jack for the change of a position of the seat base in a wheelchair. The jack has, in the lower frame (1), the driving system of the screw jack with a vertical screw (14) is fixed; and the upper end of the screw (14) is pivotally fixed in the main arms (3) of the scissors system. The driving system of the screw (14) of the screw jack is fixed to the base (11) attached slideably to the lower frame (1) in the tracks (8) in such a way that the screw (14) is permanently in a perpendicular position to the lower frame (1) and the upper frame (2) with maximum deviation of ±10°; and the base (11) is driven by the arms (17), with their first ends fixed pivotally in the main arms (3), and the other ends fixed pivotally in the base (11).