Radial spread of oil spilled on the calm water is treated using the two layer model. Governing equations of oil layer are treated as the initial and boundary value problem employing the method of characteristics. Calculation is done for region behind a position where so called the front condition is satisfied. The front condition has been proposed through laboratory experiments using saline and fresh water. The front condition is given at the nearest one to the front of all the positions where the interface lifts up, since the first instability seems to occur there. Calculation showed good agreement with the previous laboratory experiments