In the welding of structure, when a pneumatic air hammer happened to be in use (for instance, rivetting, calking, chipping) in places nearby of where the welding was going on, in some instances, a crack was produced in the weld. The authors, by using the apparatus which is illustrated in the Fig. 1, have reproduced its phenomenon in a laboratory and at the same time investigated the main factor which produces the crack, as well as the effect that is given by the vibrations to the toughness of welded joint and obtained the following summaries. (1) Generally speaking, when vibration is transmitted to the joint during its welding, the weldment would be more susceptible to cracks when the air pressure of pneumatic tool used is more higher, and the gap between the steel member is more larger, and the length of the tack weld is more smaller. (2) Crack sensitivity of the joint during its welding becomes larger under vibration caused by rivetting hammer than by chipping hammer and so becomes when fillet weld is done than when butt weld is carried out. (3) Because of such facts it is absolutely necessary to give a full attention to the vibration of joint members produced by a pneumatic tool in the case of a fillet welding. Even in a butt welding it is advisable to avoid such vibration for the first layer of weld, but from the second layer vibration will bring no unfavorable result to the weldment. (4) The cause of the development of cracks due to the vibration is not based on any metallurgical change, but it is conceivable to be a hot crack due to the mutual change that took place between joint members.