A sealing region where a valve seat and an outer peripheral surface of a enlarged diameter portion of a flare portion are contacted to each other is configured so that an end on a side opposite a combustion chamber is positioned at a position same as or on a side away from the combustion chamber than an end on a side close to the combustion chamber of a connecting region where a lid member and an inner peripheral surface of the enlarged diameter portion are connected with respect to the axis line direction.