PURPOSE: To keep wavelength accuracy in an excellent state, by connecting a digital switch to a microcomputer, measuring the displayed error of a known wavelength of a known sample at first, thereafter correcting the wavelength error by manipulating the digital switches.

CONSTITUTION: A wavelength scanning motor 20 is rotated by the command from a microcomputer 10, and a wavelength scanning optical element 30 is rotated. When a slider in a scanning mechanism is contacted with a microswitch 4 at a specified position and the switch is operated, a reference position detector 40 sends a signal (a) to the computer 10 so as to stop a wavelength scanning mechanism. A digital switch 50 is connected to the computer 10. When a reference wavelength is set, the computer 10 reads the preset value (b) of the switch 50. When the preset value (b) is zero, setting of the wavelength is performed only by the signal (a). When the preset value (b) is not zero, the wavelength corresponding to the preset value (b) is added to or subtracted from the reference wavelength. Thus the correction of the reference wavelength is performed. In this way, the accuracy of the wavelength can be kept at the excellent state.