The authors have carried out the human task simulation by a computer about the shipbuilding works in order to increase the productivity and safety so far, corresponding to the recent situation of the Japanese shipbuilding industry; decreasing of skilled workers and increasing of old workers. This report has picked up the welding works for hull construction as a main job in shipbuilding. For the typical postures such as flat, vertical, and overhead position, the human task simulation and the metabolic energy expenditure analysis have been carried out using general purpose program “Jack”. Moreover, necessary rest time to recover the fatigue has been studied, and the optimal work cycle is examined. As the result, it can be concluded that the flat position to weld, widely applied posture, is most severe in view of energy expenditure while the overhead position is most easy, and that the rule of “short breaks and often” is preferable from the viewpoint of recovery of the fatigue, especially for the works such that the consumption energy is large.