The present invention discloses a method for measuring an amount of an objective component to be measured in a sample, which comprises; preventing an electric charge in an atmosphere in a photometry chamber from transferring to the surface of a solution which generates light due to an energy variation of a substance induced by the objective component in the sample, measuring value of the light, and determining an amount of the objective component in the sample on the basis of the measured value thus obtained, and an instrument used for the method.According to the present invention, in measurement of an objective component in a sample using a spectrophotometer, problems such as between-day variation of signal values or increase of background value, etc. can be solved, and a trace component can be measured in high accuracy and high sensitivity.