This study investigated the effect of floor surface temperature on occupants' relaxation by measuring the electroencephalogram (EEG). 32 healthy male subjects participated, and the climate chamber was controlled to maintain "slightly cool" environment (PMV-0.8). The floor surface temperature was suddenly changed from 22°C to 30°C by using an electric mat. Subjects were exposed to each surface for 20 minutes. The EEG was recorded from two sites on the forehead (Fp1 and Fp2) using the Biopac MP150. A power spectral analysis revealed that the relative theta (9) and alpha (a) power increased and the relative beta (ß) power decreased when the floor surface temperature changed from 22°C to 30°C. This result shows that increasing the floor surface temperature can help occupants relax when the ambient air temperature is maintained at a slightly cool level.