Provided is a gear pump device that enables improvement in volumetric efficiency and manufacturability, and also makes it possible to ensure sealing property and to reduce drive torque. According to the present invention, a sealing mechanism is provided with an annular rubber member, an outer member, and an inner member, wherein: the inner member has, at an end of an outer peripheral wall on the side of an inner gear in the axial direction, a notch which is recessed radially inward of the inner gear so as to form, together with an axial one end face of the inner gear, a depressed part; and the outer member has an insertion part which is disposed within the depressed part and which abuts against the axial one end face of the inner gear so as to constitute a part of a sealing surface on the other side.