The authors have studied fundamentally and experimentally the explanation of the sealing mechanism of oil seals. It will be generally noticed that the suction of air occurs to the rubbing portion of the lip of an oil seal which is operating under sealing condition. For the purpose of studying sealing characteristics of an oil seal in the authors' investigations, the volume, [Qbar], of transferred lubricating oil instead of air is used as a means of estimating the sealing status.