The objective of this study was to investigate and understand brain activities and mental state of humans in response to odors. On the 10 healthy adults with no neurological abnormalities electroencephalography was performed to analyze the brain activity in subjects being exposed to the odorants with different level of pungent smell of ammonia and the resting-state brain activity after being exposed to the odorants. A questionnaire survey was performed in parallel in order to evaluate the mental state (pleasant / unpleasant) of the subjects. In the brain activity measurements, α wave band was significantly suppressed with ammonia odor even at a low level, of which impact on mental state of the subjects was not clearly expressed in the result of questionnaire survey. A significant difference in the nerve activation of β wave band at post-rest was also observed with the same conditions. The brain wave measurement is considered to be an effective tool to detect the human emotional changes, which was even not recognized by him/herself.