Experimental investigation was carried out on a swirl motion of a bottom blown bubbling jet in the presence of a top oil layer. Water was used as the lower liquid and silicone oil was used as the top oil. The kinematic viscosity of the silicone oil was changed over a wide range. The occurrence region of the swirl motion became narrow as the kinematic viscosity of the silicone oil, νo, increased. Silicone oil did not affect the period of swirl motion, Ts. As νo increased, the starting time of the swirl motion, Ts,s, increased, whereas the amplitude, A, and the damping time of the swirl motion, Ts,d, decreased. An apparent kinematic viscosity, νapp, was introduced for correlating A, Ts,s and Ts,d. Empirical equations were proposed for these quantities as functions of νapp.