An apparatus for successively making plastic bags, comprising: a pair of rollers opposed to each other; a feeding device by which two webs of panel material are superposed with each other, a body of zipper material being inserted between the webs of panel material to extend longitudinally of the webs of panel material, the webs of panel material and the body of zipper material being fed longitudinally thereof to be directed to, sandwiched between and pressurized by the rollers; and a laser beam irradiator irradiating laser beam onto the webs of panel material and the body of zipper material at a position upstream of the rollers when the webs of panel material and the body of zipper material are fed longitudinally thereof, to heat and melt the webs of panel material and the body of zipper material with the laser beam, the webs of panel material and the body of zipper material being welded to each other when being pressurized by the rollers, the apparatus being arranged to successively make the plastic bags of the webs of panel material and the body of zipper material, the apparatus being characterized by a control device connected to the feeding device and the irradiator, the feeding device being controlled by the control device so that the webs of panel material and the body of zipper material are fed intermittently at a speed predetermined to be changed by time, the irradiator being controlled by the control device and on a feedforward control basis for the laser beam to be regulated in amount, the amount being calculated from the speed predetermined, to obtain a calculated value of amount, the laser beam being regulated in accordance with the calculated value to heat and melt the webs of panel material and the body of zipper material uniformly irrespective of the speed being changed, the webs of panel material and the body of zipper material being welded to each other to obtain a welded strength kept constant. a pair of rollers opposed to each other; a feeding device by which two webs of panel material are superposed with each other, a body of zipper material being inserted between the webs of panel material to extend longitudinally of the webs of panel material, the webs of panel material and the body of zipper material being fed longitudinally thereof to be directed to, sandwiched between and pressurized by the rollers; and a laser beam irradiator irradiating laser beam onto the webs of panel material and the body of zipper material at a position upstream of the rollers when the webs of panel material and the body of zipper material are fed longitudinally thereof, to heat and melt the webs of panel material and the body of zipper material with the laser beam, the webs of panel material and the body of zipper material being welded to each other when being pressurized by the rollers, the apparatus being arranged to successively make the plastic bags of the webs of panel material and the body of zipper material, the apparatus being characterized by a control device connected to the feeding device and the irradiator, the feeding device being controlled by the control device so that the webs of panel material and the body of zipper material are fed intermittently at a speed predetermined to be changed by time, the irradiator being controlled by the control device and on a feedforward control basis for the laser beam to be regulated in amount, the amount being calculated from the speed predetermined, to obtain a calculated value of amount, the laser beam being regulated in accordance with the calculated value to heat and melt the webs of panel material and the body of zipper material uniformly irrespective of the speed being changed, the webs of panel material and the body of zipper material being welded to each other to obtain a welded strength kept constant.