[Problem] A practicable quantum meter capable of knowing the photosynthetic photon flux density at each wavelength in real time is presented.[Solution] Measurement light made incident on a dispersive element 12 by an incident optical system 13 in a spectroscope unit 1 is dispersed by the dispersive element 12, and converted to photoelectric signal on the detector 14. Each photoelectric signal at each wavelength (spectral data) is transmitted to the processing unit 2, which is a general use computer, via interface members 16 and 26. The processing unit 2, to which a photosynthetic photon flux density measurement program 4 and a special device driver 262 have been installed, calculates the distribution of the photosynthetic photon flux density at each wavelength by processing the received spectral data, and displays it on display 24.
