PURPOSE: To enable instantaneous and highly precise measurement of constituents by near- infrared rays by dispercing a reflected light into spectral lights of a number of wavelength bands and by converting them into electric signals by a detector array wherein a plurality of detecting elements are arranged.

CONSTITUTION: A measuring head unit 1 having a light-projecting device 1a inside irradiates vegetable 2 and a reflected light therefrom is made to enter a spectroscope 10 through an optical fiber 3. A diffraction grating 22 provided in the spectroscope 10 disperses the light into a plurality of spectral lights of different wavelengths and the light of each wavelength band enters each of detecting elements of a detector array 11 and is subjected to photoelectric conversion. An electric signal therefrom is inputted to an arithmetic unit 24 together with an electric signal of a reference light which is made to enter a light-sensing element 7 through the other optical fiber 3a and subjected to photoelectric conversion, and thereby a constituent of vegetable 2 is measured. Since the reflected light from the vegetable 2 is dispersed into spectral lights of a number of wavelength bands and the light of each wavelength band is converted into the electric signal by the detector array 11 made up of a number of detecting elements in this way, the constituents of the vegetable 2 can be measured instantaneously and highly precisely.