In order to suppress emission of contaminants (such as volatile organic compounds contained in building materials and furniture) in living spaces into the outside and to provide a deodorant effect, provided is a contaminant treatment agent tobe applied to the surfaces of contaminant sources (such as building materials and furniture (2)) in a living space such as a room space in a building (1), to thereby treat contaminants (such as volatile organic compounds (3)) contained in the building materials and furniture (2), the contaminant treatment agent including: stable pure water (4) which is excellent in oil component solubility and has a smaller time-dependent change in absorbance measured with a spectrophotometer under the condition of being left to stand for at least 5 hours or more than pure water including purified water, the stable pure water (4) being prepared by irradiating pure water including neutral water free of the occurrence of an electric charge imbalance with light collected by causing natural light to pass through a light collector using titanium or silicon； eucalyptus essential oil (5) that is dissolved in the stable pure water (4) and contains cineol as a main component； and a pre-selected mineral component that is added to the stable pure water (4).
