A gear pump device of this disclosure includes: a gear pump; a casing; and a seal mechanism, wherein the seal mechanism includes: an annular rubber member; an outer member; and an inner member, which has an outer peripheral wall on which the annular rubber member is mounted, the inner member fitted into an inner side of the outer member and contacting on an inner wall face of the outer shell, wherein the outer peripheral wall of the inner member is provided with a collar portion that generates a propulsive force towards the inner wall face of the outer shell of the casing by a contact pressure of the annular rubber member based on a discharge pressure of the gear pump, and the collar portion forms a pressure receiving face to increase the propulsive force as the contact pressure of the annular rubber member increases according to an increase of the discharge pressure.