A driving circuit for rotating the grating of a monochromator thereby changing the wavelength of light which is emitted from the monochromator. This driving circuit includes an operating panel for supplying wavelength data, a data converting circuit which obtains a trigonometric function value corresponding to the wavelength data when the data converting circuit receives the wavelength data, and which converts the wavelength data to angle data on the basis of the trigonometric function value, and a motor driver for rotating the grating into the angular position corresponding to the angle data.