We have developed a bio/chemiluminescence spectrometer with a cooled charge-coupled-device (CCD) detector to obtain a quantitative luminescence spectrum as the absolute number of all emitted photons at each wavelength. The integrated area of the spectrum divided by the number of reacted substrate molecules gives the quantum yield. Calibration of the absolute sensitivity of the CCD-spectrometer system was performed by using lasers and a tungsten lamp with calibrated powers as primary light standards, and calibration of the light-collection efficiency of the spectrometer with several kinds of cells for liquid samples was achieved by introducing a simple reference double-plate cell. The reference cell is not convenient for final bio/chemiluminescence measurements but is useful for the calibration because it has well-defined angular dependence of light emission, allowing accurate calculation of the light-collection efficiency. Using this CCD-spectrometer system, we re-examined the quantum yield of aqueous luminol chemiluminescence with H2O2 catalyzed by horseradish peroxidase. The quantum yield was constant for a wide range of luminol concentrations, whereas it changed and had an optimum against H2O2 concentrations. The optimum quantum yield was 1.23(±0.20)%, which is in good agreement with previously reported values.