A throw-away rotating tool includes plural protrusions formed in a part of an outer peripheral wall of a projecting coupling portion. Since the twist angle and a twist direction of the protrusions are set in such a way that with relative rotation when coupling the projecting coupling portion and erected portions together, due to the reaction force received by the protrusions from inner peripheral walls, the projecting coupling portion is pulled to the rear and side of the erected portions and the projecting coupling portion is stably held between the erected portions. Since the number of portions that require stringent dimension control and high machining accuracy can be reduced, productivity can be improved.