A high pressure tank extending between an opening side and an end side comprising: a liner having an internal space in which to seal a fluid; and a mouth piece including a cylindrical portion having an opening, and a flange connected to the cylindrical portion and projecting in a radial direction of the cylindrical portion, the mouth piece being attached to the liner, wherein: an outer peripheral surface of the flange includes a top face and a bottom face with a radially outer end of the outer peripheral surface being taken as a boundary between the top face and the bottom face; the bottom face includes an inner surface part at least partially exposed to the internal space, wherein the bottom face comprises: an annular groove hollowed toward the opening side, a connecting surface connecting the inner surface part to the annular groove and having an inclined surface, and a corner part being a corner formed between the inclined surface and the annular groove; the inclined surface is inclined toward the opening side from a radially inner end of the inclined surface to a radially outer end of the inclined surface; the annular groove includes an outer surface placed on a radially outer side relative to a most hollow part of the annular groove in an axis direction of the cylindrical portion such that a contact pressure between the corner part and the liner is higher than a contact pressure between the outer surface and the liner. a liner having an internal space in which to seal a fluid; and a mouth piece including a cylindrical portion having an opening, and a flange connected to the cylindrical portion and projecting in a radial direction of the cylindrical portion, the mouth piece being attached to the liner, wherein: an outer peripheral surface of the flange includes a top face and a bottom face with a radially outer end of the outer peripheral surface being taken as a boundary between the top face and the bottom face; the bottom face includes an inner surface part at least partially exposed to the internal space, wherein the bottom face comprises: an annular groove hollowed toward the opening side, a connecting surface connecting the inner surface part to the annular groove and having an inclined surface, and a corner part being a corner formed between the inclined surface and the annular groove; the inclined surface is inclined toward the opening side from a radially inner end of the inclined surface to a radially outer end of the inclined surface; the annular groove includes an outer surface placed on a radially outer side relative to a most hollow part of the annular groove in an axis direction of the cylindrical portion such that a contact pressure between the corner part and the liner is higher than a contact pressure between the outer surface and the liner. an outer peripheral surface of the flange includes a top face and a bottom face with a radially outer end of the outer peripheral surface being taken as a boundary between the top face and the bottom face; the bottom face includes an inner surface part at least partially exposed to the internal space, wherein the bottom face comprises: an annular groove hollowed toward the opening side, a connecting surface connecting the inner surface part to the annular groove and having an inclined surface, and a corner part being a corner formed between the inclined surface and the annular groove; the inclined surface is inclined toward the opening side from a radially inner end of the inclined surface to a radially outer end of the inclined surface; an annular groove hollowed toward the opening side, a connecting surface connecting the inner surface part to the annular groove and having an inclined surface, and a corner part being a corner formed between the inclined surface and the annular groove; the inclined surface is inclined toward the opening side from a radially inner end of the inclined surface to a radially outer end of the inclined surface; the annular groove includes an outer surface placed on a radially outer side relative to a most hollow part of the annular groove in an axis direction of the cylindrical portion such that a contact pressure between the corner part and the liner is higher than a contact pressure between the outer surface and the liner.