The selective detection of some gases was investigated by an optical chemical sensor based on laser-excited surface plasmon resonance. When the medium outside a silver thin film was changed from air to a polyethylene glycohol (n=600), the resonance angle changed from 22.20 degrees to 65.50 degrees. Moreover, as a polyethylene glycohol thin film was exposed to some gas-adsorbing organic materials such as hydrocarbons, aldehydes, alcohols etc., the shifts of the incident angle were observed. The sensitivity increased with an increasing number of carbons in the alcohol. In contrast, the shifts were rarely recognized when lower hydrocarbons were introduced.