A jack defining a portable accessory for replacing a motor vehicle wheel; the jack comprising: - resting means adapted to be placed on the ground; - a scissor-like pantograph comprising a first and a second arm (11,12) hinged to each other about a first axis and comprising a first and, respectively, a second lower end portion; said first and second lower end portions (14,15) being supported by said resting means and being movable with respect to each other along a horizontal direction orthogonal and skew with respect to said first axis; - a supporting member coupled to an upper end of said pantograph and adapted to be placed underneath said motor vehicle; and - a screw/nut-screw device, which connects said first and second arm (11,12), is adapted to be operated to move said first and second lower end portions (14,15) close or away along said horizontal direction and comprises: a) a screw, which extends along a second axis orthogonal and skew with respect to said first axis and is coupled to said second lower end portion in such a manner that the screw is rotatable about said second axis and is rotatable about a third axis parallel to said first axis, and b) a nut-screw, which is screwed on said screw about said second axis and is coupled to said first lower end portion in such a manner that the nut-screw is angularly fixed about said second axis and is rotatable about a fourth axis; said fourth axis being parallel to said first axis and being arranged higher than said third axis; characterized in that: - said second arm is shorter than said first arm and comprises an upper end portion hinged to an intermediate portion of said first arm about said first axis; - said supporting member is supported only by the upper end of said first arm and is rotatable about a fifth axis with respect to said upper end; - said fourth axis is fixed with respect to said first lower end portion. - resting means adapted to be placed on the ground; - a scissor-like pantograph comprising a first and a second arm (11,12) hinged to each other about a first axis and comprising a first and, respectively, a second lower end portion; said first and second lower end portions (14,15) being supported by said resting means and being movable with respect to each other along a horizontal direction orthogonal and skew with respect to said first axis; - a supporting member coupled to an upper end of said pantograph and adapted to be placed underneath said motor vehicle; and - a screw/nut-screw device, which connects said first and second arm (11,12), is adapted to be operated to move said first and second lower end portions (14,15) close or away along said horizontal direction and comprises: a) a screw, which extends along a second axis orthogonal and skew with respect to said first axis and is coupled to said second lower end portion in such a manner that the screw is rotatable about said second axis and is rotatable about a third axis parallel to said first axis, and b) a nut-screw, which is screwed on said screw about said second axis and is coupled to said first lower end portion in such a manner that the nut-screw is angularly fixed about said second axis and is rotatable about a fourth axis; said fourth axis being parallel to said first axis and being arranged higher than said third axis; a) a screw, which extends along a second axis orthogonal and skew with respect to said first axis and is coupled to said second lower end portion in such a manner that the screw is rotatable about said second axis and is rotatable about a third axis parallel to said first axis, and b) a nut-screw, which is screwed on said screw about said second axis and is coupled to said first lower end portion in such a manner that the nut-screw is angularly fixed about said second axis and is rotatable about a fourth axis; said fourth axis being parallel to said first axis and being arranged higher than said third axis; - said second arm is shorter than said first arm and comprises an upper end portion hinged to an intermediate portion of said first arm about said first axis; - said supporting member is supported only by the upper end of said first arm and is rotatable about a fifth axis with respect to said upper end; - said fourth axis is fixed with respect to said first lower end portion.