[en] In helium discharge plasmas, the relative emission intensities of the fine-structure transitions belonging to the HeI 2"3S–2"3P transition can be affected by radiation reabsorption. Since the magnitude of the reabsorption depends on the density and temperature of the 2"3S metastable atoms, their density can be determined by measuring the 2"3S–2"3P emission line shape using a high wavelength-resolution spectrometer. In this study, the applicable conditions of the method in terms of the opacity and line broadening are revealed, and possible causes of errors in the measurement, i.e. spatial distributions of the density and temperature and the effects of external magnetic and electric fields, are investigated. The effect of reabsorption under an external magnetic field is experimentally confirmed using a glow discharge plasma installed in a superconducting magnet. (paper)