Entrapment of slag into a metal bath at the slag-metal interface in a ladle or an iron bath of smelting reduction process is of practical importance. Cold model experiments were performed to make clear the critical flow rate of injected gas causing the entrapment of slag under bottom gas injection. Water and salt water were used as models of metal, while liquid paraffin, three kinds of silicone oils with much different kinematic viscosity, and n-pentane as models of slag. Air was injected through a centric bottom nozzle into the bath. Entrapment of slag was judged from visual observation. An empirical correlation for the critical gas flow rate was derived as a function of density and kinematic viscosity of slag and metal, interfacial tension between slag and metal, bath diameter, inner diameter of nozzle and bath depth. This correlation could predict the critical gas flow rate for previous hot model experiments as well, provided that the flow patterns in the baths were similar to the present case.