Multi-thermal equilibrium (MTE) method, which has been employed for measurement of argon or helium temperature in thermal plasma, was applied to a non-thermal helium plasma, in an electron cyclotron resonance (ECR) helium plasma. Relative densities of excited atoms in the vicinity of substrate were measured based upon an optical emission spectroscopy. Relation of relative densities of excited atoms with the energy level, Boltzmann plot, showed a quasi-thermal equilibrium, e.g. the temperature could be determined from the slope of the Boltzmann plot. The temperature was measured by changing negative bias voltage applied to substrate, distance from surface of substrate, discharge gas pressure and substrate material. The temperature was in the range from 3600 to 4500 K.