PURPOSE: To obtain a uniformly press-welded product without requiring skill and intuition by performing gas pressure welding under pressure control according to the pressure control program made by each diameter of reinforcing bars or resembling materials and each diameter of the hydraulic ram of a pressurizing device.

CONSTITUTION: The operator turns on first a power source switch, sets the switch for selecting the kind of the diameter of reinforcing bars 1, 1' and a switch for selecting the cylinder diameter of a hydraulic ram 4 at prescribed values and heats the press welding point 2 of the bars 1, 1'. When the pressure attains the set value of a high range pressure, said pressure is detected by a pressure sensor 6 and a control device 10 stops the hydraulic pump of a pressurizing device 7. When the pressure of the ram 4 decreases down to a lower limit pressure, the hydraulic pump of the device 7 is again operated by the signal from the device 10 to increase the pressure up to the set value of the high range pressure. The pressure decrease-reincrease in a high range 12 are thus repeated by a prescribed time. When the pressure of the ram 4 decreases to a set value 14 of a low range, the device 7 is operated again to increase the pressure up to the set value 14A of the high range pressure. The pressure decrease-reincrease are repeated several times by which the bars 1, 1' are press-welded.