A micellar slug for use in the recovery of oil, the slug containing a hydrocarbon, an aqueous medium, a surfactant, and a cosurfactant. The surfactant contains as essential components (a) at least one alpha-olefin sulfonate having 10 to 30 carbon atoms and (b) at least one ethoxylate selected from the group consisting of polyoxyethylene alkyl ethers and polyoxyethylene alkylphenyl ethers in a weight ratio of component (a) to component (b) of 8/2 to 3/7. The micro-emulsion can be formed from this micellar slug in a wide composition range. Furthermore, this micellar slug has an excellent salinity tolerance and hard-water resistance.