Tank and manufacturing method thereof

A tank comprising:

• a liner;
• a mouthpiece including an annular recess portion and a collar portion which is formed continuously with the recess portion;
• and
• an FRP layer including a hoop layer and a helical layer formed by winding fibers around an outer periphery of the liner,
• wherein at least an innermost helical layer is a smooth helical layer, and
• the smooth helical layer is formed by setting a predetermined number of N points, wherein N is 5 or more, equally arranged in a peripheral direction as equally divided reference positions, winding the fibers around the outer periphery of the liner so that the fibers successively pass the reference positions by helical winding, and then successively winding the fibers around positions shifted in the peripheral direction by a predetermined shift number s, wherein the predetermined shift number s is greater than one, so that each fiber is disposed immediately adjacent to a previously wound fiber, with substantially no gap between adjacent fibers.