PURPOSE: To properly set the light-measuring timing for the high-speed phenomenon to be observed and enable it to be measured, by providing a plurality of openings on the circumference of a light-scanning disk at equal intervals, arranging it near the incident slit in a spectroscope, and forming the intervals between the openings so as to be equal to the height of the incident slit.

CONSTITUTION: The light passing through an analysis sample 11 passes through light-passing slots 42 provided o radiuses r of a light-scanning disk 43 and reaches the incident slit 21 in a spectroscope 2, where said light is dispersed according to the wavelengths by a diffraction grating 24, and the lights of wavelengths λ1Wλn are lined up on the screen of an image detector 3. The light allowed to reach the incident slit 21 by the high-speed revolution of lthe light-scanning disk 43 is scanned by each light-passing slot 42 which moves downward over th incident slit 21 at high speed from the point of time when the slot 42 has reached the uppermost end of the slit 21 in the height direction thereof.