The objective is to enable the formation of an outer circumferential layer by a filament winding process to be performed appropriately, and to achieve the weight reduction of a gas tank by reducing the weight of a mouthpiece. The gas tank comprises a mouthpiece attached to an end of a tank main body in its axis direction, and an outer circumferential layer formed over an outer circumferential surface of the tank main body and an outer circumferential surface of the mouthpiece by a filament winding process. On the end of the tank main body in the axis direction, a recess section, in which an outside end surface of the mouthpiece is embedded in the inner side from an outside end surface of the outer circumferential layer, is formed.