To provide a gear pump for a viscous fluid formed as an internal bearing type self-lubricating with the conveyed viscous fluid and provided with a shaft sealing device that can impede the leakage of this fluid to the outside.
A shaft sealing device SS for a driving shaft 4S piercing a side plate 2 of a gear pump is composed of labyrinth seal mechanism LS with a labyrinth cylinder 8 surrounding the driving shaft 4S and forming an annular clearance along the peripheral surface of the driving shaft 4S to perform sealing function and mechanical seal mechanism MS performing a sealing function by mutual sliding contact between a rotating ring disposed outside of the labyrinth seal cylinder 8 and fixed to the driving shaft 4S, and a fixed ring fixed to the side of the side plate 2. The leakage of a viscous fluid is impeded by both seal mechanisms, and internal pressure can be kept constant so as to impede inflow of air from the outside.