Low-electron-temperature ECR plasma with high electron density was realized under the mirror magnetic field configuration in the H-2 and the Ar/N-2 plasma. Especially, the electron temperature was observed to be less than 2 eV in the Ar/N-2 plasma. it was found from the calculation of particle and power balance in steady state that the decrease in the electron temperature observed in the Ar/N-2 plasma was due to the effect of the magnetic-mirror confinement of the N-2 plasma. Furthermore, our calculated results suggest that the effect of magnetic-mirror on the decrease in the electron temperature depends on the collisional cross section between electrons and neutral particles.