PURPOSE: To enable the high precision correction by the use of an easily available element by carrying out the correction based on the signal in connection with a reference light and a measuring light obtained by actually measuring two standard samples.

CONSTITUTION: Two standard samples SS1, SS2 having a flat spectroscopic transmission are prepared. Through the first standard smaple SS1, a signal eR1 relating to the reference light R is added to a sample hole circuit SHR. Then, through the second standard smaple SS2, a signal eM2 relating to the measuring light is added to a sample hold circuit SHM. From these, eM2/eR1 is obtained, whereby the change in sensitivity with respect to a predetermined span can be corrected. The correction in the case of a measuring material being zero is to be carried out by obtaining the ratio of the signal eM1 relating to the measuring light M and the signal eR1, eM1/eR1. Thus, the high precision correction can be done.