Abstract: An arm unit for use in a pantograph type jack wherein a main driving cam having an effective cam face constituted with an arc of a circle is formed to one side plate at the mounting end of a U-cross sectioned arm on the side of a bearing member with the effective cam face being protruded forwardly and upwardly from the mounting hole for the shaft bearing member, whereas a driven cam with a relatively moderate and long cam face different from the shape of the main driving cam is formed to the other of the side plates such that, when a pair of opposed arms is an identical shape pivoted to the bearing member are rotated in the directions opposing to each at an equal angular velocity, the main driving cam and the driven cam for each of the arms are faced and brought into engagement under contact with each other.