To improve sealability of a seal member in a tank.

This tank includes: as resin liner having a cylindrical opening part formed of resin and a projection part formed on an outside surface of the opening part and storing a predetermined liquid inside; an expansion restraining member arranged so as to abut on the outside surface of the opening part and restraining the expansion of the opening part, that is, the expansion restraining member suitable for the projection part and having a recessed part for enhancing abutting performance with the opening part; a mouthpiece inserted into an opening part of the rein liner; and the seal member arranged between the mouthpiece and the resin liner, forming a seal line and arranged so that the seal line does not pass through a root part of the projection part.