We constructed a system in which a newly developed pulse excited YAG laser oscillator of 1.5 kW average power and an integration optics were incorporated, together with two cw YAG laser oscillators of 2 kW average power, a 6-axes articulated robot and a PC. The integration optics summed up three laser beams transmitted through optical fibers and condensed to a focused spot at the focusing point. We could obtain the following results using the system. (1) The focused spot diameter for good welding quality with the integration optics could be available without changing the distance between the specimen and the optics. (2) The delay time between the pulse excited YAG laser beam and the rectangularly modulated cw YAG laser beams should not exceed the time of keyhole formation with the cw laser beams to obtain the maximum penetration depth. (3) The deepest penetration depth and the largest volume of welding could be obtained by the overhead position welding compared with the flat, horizontal and vertical position welding.