PURPOSE: To prevent seizure at the time of starting operation by feeding a lubricating oil from outside into a space between a seal ring and a seal plate for gear driving shaft immediately before starting operation of a gear pump.

CONSTITUTION: A cover 8 is demounted immediately before starting operation of a gear pump, and a lubricating oil is fed between a seal ring 6 and a seal plate 4 for a gear driving shaft 7. Upon starting of the gear pump operation, the lubricating oil spreads over the sealed surface, and matters to convey arrive in sequence at the sealed surface from the starting of operation to self-lubrication. Therefore an improved precision of the seal is still to prevent a seizure at the time of starting operation.