Floor-supply air-conditioning system with Coanda effect is introduced to a theater hall. This system supplies air from floor diffusers set beneath the seat backrest in front of occupants. Supplied air moves upward along the backrest board inducing the circumambient air resulted in airflow temperature difference reduction. Thermal environment was measured under various heat load conditions in the theater hall. Supply air flow along and radiation effect from the seat backrest in front were confirmed using the floor-supply system. In summer, vertical temperature differences in the hall were diminished in the case under condition (i), multiple air conditioners were in operation. In winter, there were horizontal temperature differences at the lower part caused by down draft from a stage area. Also, the authors carried out the ventilation performance evaluation by the local mean age of air in summer and winter. The horizontal distribution of local mean age of air in the breathing levels was little. In summer, it was 22.1 - 33.2 minutes and in winter, it was 55.8 - 62.4 minutes. The efficacy of the ventilation performance of this system was confirmed.