One sided automatic welding technique has broadly been prevailed in our shipyards. This technique not only enables omission of turning from side to side but also improves production processes of ship structures. But it is observed that cracking occurs in the center of weld metal near the end of weld joints. So far, complete preventing methods which do not violate production processes seem not to be found. In this paper a preventing method is proposed. In the process of one sided automatic welding, restraint force (1030 ton) should be loaded on the two devided tab plate and furthermore the tack weld near the end of weld joints should be removed (extend over 2 m). Experimentally it is confirmed that the end cracking has not appeared and it seems to be very useful preventing method. In order to analyse this preventing method, authors have proposed calculation technique which differs from the ordinary welding thermal stress analysis such that the conditions of calculation are simulated to be more realistic welding procedures. And special attention is paid to that yield phenomenon of tack weld and strength of electro magnets which sustain welded plates are considered.