PURPOSE: To enable exact quantitative determination of a low concn. element having a very weak emission spectrum by measuring the profile of the emission spectrum plural times, and adding the same thereby matching exactly the monochromator to the peak wavelength of the spectral line.

CONSTITUTION: The grating 4 of a monochromator 3 is controlled by a wavelength driving circuit 5 which responds to a control circuit 8, and the monochrome light of the wavelength increasing successively near the wavelength of an analysis line is detected with a photoelectric detector 6 and is written into the address of a storage cell 9 assigned by the circuit 8. The similar operation is reiterated upon ending of the writing up the prescribed address and the addition profile is stored via an adding an calculating element 11 and an adder 7 in a cell 9. If the same operation is repeated by n-times, the noise component in the addition profile decreases in proportion to 1/n1/2. If the address of the peak wavelength is read from the cell 9 and the monochromator is controlled with the circuit 5, the monochromator is matched with the peak wavelength of the spectral line and the low concn. element having the very weak emission spectrum is exactly quantitatively determined.