A shaft-sealing device includes an outer ring for receiving a shaft sleeve to rotate therewith. A magnetic-field generating unit is mounted the around rotary shaft and includes inner and outer magnetic rings. The outer magnetic ring is mounted in the outer ring. A gap is defined between an inner periphery of the outer magnetic ring and an outer periphery of the inner magnetic ring and is filled with a magnetic liquid. One or both of the inner and outer magnetic rings provide magnetic force to retain the magnetic liquid in the gap. A seal ring is mounted between the outer magnetic ring and an end of the outer ring. A lip extends from an inner periphery of the seal ring toward or away from the end of the outer ring and is at a non-parallel angle to a radial direction of the seal ring to form a seal around the rotary shaft.