We performed numerical analyses on the optimal discharge angle of a 4-way cassette air conditioner, which is mounted on the ceiling of the classroom, by using thermal comfort and ventilation indices. We analyzed the effective draft temperature, the local mean age and the local mean residual life time with respect to the variation of the air discharge angle of the air conditioner in all occupied zone. Air diffusion performance index, local supply effectiveness and local exhaust effectiveness were used to investigate the optimal value of air discharge angle. From the results, we found out that the optimal discharge angle of the air conditioner is about 40 degrees in the present model. Keywords：Air diffusion performance index, Effective draft temperature, Local exhaust effectiveness, Local supply effectiveness, Ventilation effectiveness