It is important to predict the stress driven hydrogen induced cracking at the weld joint on the basis of computational mechanics from the view point of engineering problem. In this study, On the basis of proposed numerical analysis, behaviors of hydrogen diffusion and concentration during cooling process of y-grooved weld joint were analyzed and the mechanism of hydrogen induced cracking was investigated. One of authors has been proposed a multiplication method which magnifies the hydrogen driving term in the diffusion equation to realize correctly hydrogen concentration behaviors. In this study, the behaviors of hydrogen diffusion and concentration for the model of y-grooved weld joint was analyzed by combining a multiplication method with the coupled analyses of heat transfer – thermal stress – hydrogen diffusion. As a result, hydrogen was found to diffuse from weld metal to base metal through HAZ (Heat Affected Zone), and concentrate at the position of blunt angle side of weld groove bottom. It was found that hydrogen concentrates at the position of the local maximum value of hydrostatic stress gradient. This analytical result was found to well predict the actual hydrogen induced cracking of the y-grooved weld joint. Using this method of analysis, prediction of hydrogen induced cracking becomes possible.