PURPOSE: To realize an accurate automatic calibration for the wavelength by measuring each spectrum focus position of the plural bright lines featuring the known wavelengths for input to the computer and then memorizing the error to the logical spectrum focus position set previously.

CONSTITUTION: Mercury lamp 2 produces a number of known wavelength bright lines, and thus optical detector 9 has scanning along spectrum focus surface 8 to detect the bright line positions in sequence for input and memorization into computer 10. The signal showing the position of detector 9 is indicated by the crossing number of the magnet scale and the like which is provided to cover the range of surface 8. As the logic value of the spectroscope is memorized in computer 10, a comparison is given between the logic value and each spectrum focus position measured by detector 9 to obtain the difference between them. This difference is then memorized in computer 10 as the value to receive the calibration. In such way, an accurate and automatic calibration becomes possible for the wavelength.