Modernization of shipbuilding industry cannot be achieved without establishing such highly standardized and specialized production processes as those of mass production. Systematically 'composed mechanization of automatic welding, for example, attained by essential application of numerical control technique to the drawing, fabricating and assembling works, will show the most, effective way of modernization. On this basis, shipbuilding operations go on as smoothly as they do in mass production, with production control through electronic data processing. The authors have established wide range of computer system, improving production system. The computer system in the field of production management, named “COSPAC”, involves overall production planning programs from master schedule to detail operation schedules. This paper relates with computer program for production planning, “LOTAS”, which is a part of COSPAC. The authors lay great emphasis on repetitive computer simulation in order to increase reliability of production planning, efficiency of production and profitability of shipbuilding.