A gear pump apparatus includes a gear pump and a sealing mechanism which includes an annular rubber member, an outer member, and an inner member. One of the outer member and a casing of the gear pump apparatus has a contact member located outside a portion of the outer member which contacts the gear pump in a radial direction of the gear pump. The contact member is placed to create a physical contact between the outer member and the casing to absorb a part of force by which the outer member is pressed against the gear pump. This results in a decrease in pressure acting on an area of contact between the outer member and the gear pump, which leads to a drop in resistance to sliding between the gear pump and the outer member, thus decreasing a loss of torque required for the pumping operation of the gear pump.