The light trespass problem still has not been solved for a long time because of the complex influences of the reflection or overlap lights. To solve the light trespass problem, the assessment of the characteristics of the spilt light carry out firstly. In this study, we estimated the light trespass reducing performance of various load lightings using by RELUX lighting simulation program. In the simulation program, we designed the lighting environment according to the streetlight installation guideline by the ministry of land, transport and maritime affairs. And we estimated the light trespass caused by roadway lighting from first floor position to fourth floor position. And we compare the light trespass caused by back light of the roadway lighting with the light trespass caused by forward light. Then we estimated the effect of light trespass by improving the light distribution of roadway lighting from Backlight rating 3 to Backlight rating zero. From the research, we find that the light trespass caused by roadway lighting maximize at the first floor position of the house. And the light trespass contribution rate by backlight and forward light is 66 percentage and 33 percentage respectively. And we estimated that the light trespass can be decreased about 40 % by improving the backlight. The result derived from this research will be helpful to establish the light pollution reduction plan to minimize the adverse effect by the spilt light from the roadway lighting. Keywords ： Light Trespass, Lighting Simulation, RELUX, Roadway lighting