A welding system comprising: a pair of welding objects butted against each other with a groove between the welding objects; a laser head configured to emit laser light; a welding torch configured to apply a voltage to a welding wire to perform arc welding; being characterised by: main control device comprising a beam diameter deciding unit, a beam diameter adjusting unit, and a welding position adjusting unit, a distance meter configured to measure the shape of the groove of the welding objects; a groove width detecting unit configured to detect the groove width W1 on the basis of the measurement result by the distance meter; the beam diameter deciding unit configured to decide the processing point beam diameter W on the basis of the groove width W1 detected by the groove width detecting unit, such that the laser light has a processing point beam diameter larger than a width of the groove, so that the laser light melts a bottom part and side parts of the groove and the melted inside of the groove is arc-welded by the welding torch to form a weld bead; the beam diameter adjusting unit configured to decide the height in the Z-axis direction of the laser head; the welding position adjusting unit configured to decide the length LL between the processing points P1 and P2 on the basis of the processing point beam diameter W decided by the beam diameter deciding unit; an interval adjusting mechanism configured to adjust an interval between the laser head and the welding torch according to the instructions provided by the welding position adjusting unit 430; and a moving unit configured to move the laser head and the welding torch with the interval there between adjusted, along the groove. a pair of welding objects butted against each other with a groove between the welding objects; a laser head configured to emit laser light; a welding torch configured to apply a voltage to a welding wire to perform arc welding; being characterised by: main control device comprising a beam diameter deciding unit, a beam diameter adjusting unit, and a welding position adjusting unit, a distance meter configured to measure the shape of the groove of the welding objects; a groove width detecting unit configured to detect the groove width W1 on the basis of the measurement result by the distance meter; the beam diameter deciding unit configured to decide the processing point beam diameter W on the basis of the groove width W1 detected by the groove width detecting unit, such that the laser light has a processing point beam diameter larger than a width of the groove, so that the laser light melts a bottom part and side parts of the groove and the melted inside of the groove is arc-welded by the welding torch to form a weld bead; the beam diameter adjusting unit configured to decide the height in the Z-axis direction of the laser head; the welding position adjusting unit configured to decide the length LL between the processing points P1 and P2 on the basis of the processing point beam diameter W decided by the beam diameter deciding unit; an interval adjusting mechanism configured to adjust an interval between the laser head and the welding torch according to the instructions provided by the welding position adjusting unit 430; and a moving unit configured to move the laser head and the welding torch with the interval there between adjusted, along the groove. main control device comprising a beam diameter deciding unit, a beam diameter adjusting unit, and a welding position adjusting unit, a distance meter configured to measure the shape of the groove of the welding objects; a groove width detecting unit configured to detect the groove width W1 on the basis of the measurement result by the distance meter; the beam diameter deciding unit configured to decide the processing point beam diameter W on the basis of the groove width W1 detected by the groove width detecting unit, such that the laser light has a processing point beam diameter larger than a width of the groove, so that the laser light melts a bottom part and side parts of the groove and the melted inside of the groove is arc-welded by the welding torch to form a weld bead; the beam diameter adjusting unit configured to decide the height in the Z-axis direction of the laser head; the welding position adjusting unit configured to decide the length LL between the processing points P1 and P2 on the basis of the processing point beam diameter W decided by the beam diameter deciding unit; an interval adjusting mechanism configured to adjust an interval between the laser head and the welding torch according to the instructions provided by the welding position adjusting unit 430; and a moving unit configured to move the laser head and the welding torch with the interval there between adjusted, along the groove.