A device for changing the stroke of a caulking machine having a table on which is placed an article having a plurality of rivet shanks, and a rivet head forming tool. The table is adapted to be moved back and forth and right and left to bring the rivet shanks into the position right under the rivet head forming tool one after another. The rivet head forming tool is driven by a motor and moved up and down by a cylinder to form a rivet head at the top of each rivet shank. The device for changing stroke has a plate member movable up and down together with the rivet head forming tool and stopper pins having their top ends secured to the plate member. An annular plate is mounted outside a rotary shaft of the rivet head forming tool so as to be freely rotatable at a predetermined position. A plurality of pins having lengths corresponding to the lengths of the rivet shanks of the article are secured at equal intervals to the annular plate to extend upright. The pins are adapted to be brought into abutment at the top ends thereof with the bottom ends of the stopper pins to restrict the down stroke of the rivet head forming tool.