Abstract: The invention is an improved jack having a safety device for preventing the jack from collapsing under a heavy load. The improved jack includes a linkage movable from a lowered position to an elevated position for engaging and elevating a load, an actuating mechanism connected to the linkage for moving the linkage toward the elevated position upon actuation of the actuating means, and the safety device blocking mechanism for blocking movement of the linkage from the elevated position to the lowered position. The linkage is supported and disposed for pivotal movement between two side plates. The blocking mechanism provides an abutment across the two side plates to limit the movement of linkage. The blocking mechanism features a block fixed to the linkage. The block has horizontal bores passing therethrough. A lock rod is inserted through the horizontal bores to provide an abutment across the two side plates when the linkage is elevated, thereby preventing the linkage from unintentionally lowering.