The purpose of this study is to verify differences of occupants' attention affected by the indoor temperature and to examine occupants' attention changed with time. To achieve the purpose, a temperature was controlled according to two levels in a chamber. A slightly cool level was 20.5°C (PMV -1) and a neutral temperature level was 24.0°C (PMV 0). Brain waves were measured on seven spots of the frontal lobe, the temporal lobe and the parietal lobe. Brain waves were sorted out according to the frequency using power spectrum analysis, and then that were used for the attention ((SMR+M. Beta)/Theta) analysis and the relative power analysis. As the result, a slightly cool thermal environment would be better than a neutral thermal environment for the occupants who perform the attention task.