A transition piece of a combustor which is sustainable even under conditions of more severe thermal environments is provided in the present invention. A transition piece of a combustor is provided with a cylindrical trunk main body, a cylindrical exit trunk part which is joined to the downstream end of the trunk main body and which cooperates with the trunk main body to constitute a trunk part, and an inner flange which extends from the downstream end part of the exit trunk part toward the outer periphery side of the exit trunk part. The exit trunk part and the inner flange are of an one-piece product. On the exit trunk part, there is formed a groove, and there is formed a cooling fluid passage which opens at the groove.