In a stern tube sealing device, slurry contained in outboard water is suppressed from depositing on sliding portions between a wall surface of a seal ring chamber or an outer peripheral surface of a propeller shaft and a sealing surface of a seal ring. A first seal ring (121) is arranged in a first seal ring chamber (111), and a second seal ring (122) is arranged in a second seal ring chamber (112) which is formed at an inboard side of the first seal ring chamber (111). The first seal ring chamber (111) and the second seal ring chamber (112) are in communication with each other through the sliding portions between a wall surface of the first seal ring chamber (111) and a sealing surface (121a) of the first seal ring (121). And, the outboard water is filled into the first seal ring chamber (111), and clear water is filled into the second seal ring chamber (112).