A stern tube seal wherein a rotatable and slidable ring in close contact and sliding with respect to a fixed slidable ring secured to a stern frame side is loosely slipped over a rotating shaft (stern tube), and a packing having a shape of approximately funnel is interposed between said rotatable and slidable ring and a housing secured to said rotating shaft, whereby relative displacement of said rotating shaft with respect to said stern frame is not propagated to said rotatable and slidable ring by elasticity of said packing.