Helium (He) line emissions have been utilized to measure the electron density (ne) and temperature (Te), and validity checks have been conducted in various linear devices. In this study, we performed optical emission spectroscopy (OES) of He line emissions in the linear plasma device Magnum-PSI, where the used density range was 1-8 × 1020 m-3, which was much higher than those used until now. We observed nine line emissions in the wavelength range of 388-728 nm and deduced ne and Te based on comparisons with a collisional radiative model. From the variation of the difference between the experiments and calculations, the joint probability distribution of ne and Te was deduced. We will discuss the effect of radiation trapping, in particular, based on comparisons between OES measurement results and Thomson scattering measurements.