PURPOSE: To make it possible to safely and easily measure a radioactive specimen or a large specimen too bulky to be accommodated in a specimen chamber, by guiding luminous flux to the separate specimen chamber provided outside the specimen chamber of a spectrophotometer from said specimen chamber by using a light guide and converting an optical signal to an electric signal by a detection part other than the detector of the spectrophotometer.

CONSTITUTION: A spectrophotometer has a light source, a photometer, a specimen chamber 2, a detector and a data processing part and light is guided to the photometer from the specimen chamber 2 by using an optical fiber 6 as a light guide to be converted to monochromatic light which is, in turn, separated into specimen light 3 and reference light 4 by a chopping mechanism while both lights are guided to a separate specimen chamber 8 and a reference specimen chamber 9 provided outside the specimen chamber 2. An optical element 10 for converting light to parallel light 11 is provided to the chamber 8, 92 in the vicinity of the emitting end surface 9 of the optical fiber 6 and an element 13 for condensing the transmitted lights from a specimen 12 and a reference specimen 93 to an optical fiber 14 is further provided. The respective transmitted lights are converted to electric signal by a separate detector 90 to be recorded or displayed by using the signal processor of the spectrophotometer 1. By this method, the spectral measurement of a radioactive specimen or a large specimen (e.g., a glass plate) not received in the specimen chamber 2 can be also safely and easily measured.