The use of overset concept for the unstructured grid method is relatively unexplored. However, the overset approach can extend the applicability of the unstructured grid method for real engineering problems without much efforts in code developments. Multiple moving-body problem is one of those applications. In this paper, the overset unstructured grid method is applied to a flow simulation about an experimental supersonic airplane separation from a rocket booster. Two unstructured grids, each of which covers the airplane and the rocket booster respectively, are used for the simulation. The grid around the rocket booster moves with time in the stationary grid about the airplane. The computed result clearly simulates the shock wave patterns between two bodies. Comparisons with the experimental results show good agreements in the lift and pitching moment coefficients of the airplane and booster during the separation process.