Oil mist leakage through a viscoseal under laminar flow conditions is experimentally investigated in relation to the pressure difference across the seal. The leakage cannot be confirmed when the pressure difference is smaller than a limiting value of around 40 to 50 percent of the maximum pressure difference generated by the pumping action of grooves. At a higher pressure difference, oil mist leakage appears, and the leakage rate increases in proportion to the increase in the pressure difference. Under the assumption that oil mist leakage occurs due to the backflow of pumped air in the grooves, the route of oil mist flow is studied. A correlation between the velocity distribution and the occurrence of oil mist leakage has been recognized. The limiting pressure difference where no oil mist leakage occurs can be obtained within a certain degree of accuracy from the velocity distribution in a groove with certain assumptions.