In various welded structures welding will be done more or less under external restraint. The intensity of restraint will influence on the reaction stress and shrinkage in welded joints and especially on the first pass weld cracking. However the informations on the intensity of restraint in a welded joint which is related to reaction stress are left yet quantitatively unknown. As for the intensity of restraint the following definition was adopted ; intensity of restraint is equal to average value of transverse stress along weld line necessary to produce average elastic change of root gap of unit magnitude along the weld line. The intensity of restraint thus defined will be able to be obtained by using elastic theory for restrained specimens of simple shape, but it will be difficult for more complicated specimens and practical welded joints. In order to measure the intensity of restraint the authors designed a measuring apparatus called “Restraint Meter”. Principle of the restraint meter is as follows : Separating force is applied to both beveled edge planes of a joint before welding by the action of a wedge pushed into root gap of the joint and then the change of root gap is measured by displacement gauges. The separating force per unit change of root gap will be increased with the increase of restraint. The intensity of restraint can be obtained from the separating force.