In an apparatus for successively making plastic bags, a laser beam irradiator (5) irradiates laser beam (6) onto webs of panel material (2) and a body of zipper material (3) to heat and melt the webs of panel material (2) and the body of zipper material (3) with the laser beam (6). The webs of panel material (2) and the body of zipper material (3) are fed intermittently at a speed V predetermined to be changed by time T. The irradiator (5) is controlled on a feedforward control basis for the laser beam (6) to be regulated in amount Q, the amount Q being calculated from the speed V predetermined, to obtain a calculated value of amount Q. The laser beam (6) is regulated in accordance with the calculated value to heat and melt the webs of panel material (2) and the body of zipper material (3) uniformly irrespective of the speed V being changed.