A jack includes an outer tube, an inner tube and a lifting device. The outer tube has a positioning member and a driving rod connected thereto. The positioning member is positioned by an object and the driving rod is driven by a crank. The inner tube is located in the outer tube and the lifting device is located on the top of the inner tube so as to lift the outer tube. The positioning member is located on the top of the outer tube and the driving rod is located below the positioning member. The positioning member is positioned by a pivotal device which pivots the positioning member to a desired angle and positions the positioning member.