It is the general practice under the normal conditions, to adopt the simultaneous welding by multiple workmen for block butt-joint of ship assembly. In order to clarify the characteristic of this method, we have ascertained the characteristic of constraining force of material such as strong-back restraining the shrinking of butt-joint. We have then worked out an approximate formula in respect to how the value of constraining force of block butt-joint being effected and have analized the variance resulting from the increase in number of welders in accordance with the said formula. The result of analysis thus obtained through experiments on the actual ship assemblies are as follows : - 1. The characteristic of the constraining force of strong-back varies considerably according to its type, particularly the strong-back of wedge-chocking type showing the slipping and greatly reducing the constraining force when the shrinkage of bntt-joint exceeds over 0.3 m/m. 2. The constraining force of keel-block being very small-under 1/200 of those of the strong-back, it may be ignored when the constraining force of welded bntt-joint is discussed. 3. As the number of welder team increases, the constraining force at the beginning of welding decreases. For the ordinary block butt-joint, the constraining force in case of one welder being employed for 2 pieces of the material, decreases as low as 60-70% of the case of one welder for 5 pieces and its value seems to be around 10 kg/mm²/mm. 4. In case the constraining material is of such type as showing slip phenomenon, the slipping movement of block body increases rapidly with the constraining force decreasing suddenly as soon as the first layer of welding is started. For the ordinary block butt-joint, in case of one welder being employed for 2 pieces of the material, its constraining force decreases quite rapidly as the first welding layer reaches about 400 m/m. 5. As the characteristic of simultaneous welding of butt-joint by multiple welders varies according to the type and character of the constraining material, the advantage can be attained to the fullest extent by using the proper constraining material, i. e. the less the constant value of constraining force and the faster slipping, the better its result. In practical application it is considered to be most desirous to get the best result by using the wedge-chocking strong-back placed diagonally. While in Report No. 1 is described regarding the research and study of costraining materials and the simultaneous welding by multiple welders, as well as its theoretical analysis, we will observe and report the result of our experimental researches of the actual ship assembly in order to verify its theoretical analysis, in Report No. 2.