Electron Cycrotron Resonance argon plasma was investigated by optical methods. A luminosity distribution of Ar‐II was imaged through an interferometer filter and the luminous arch hill was observed near the ECR region, where the plasma was resonantly generated. Absorption spectroscopy using laser diode was applied to neutral metastable argon. The typical value of the density amounted at 1010particles per cm3. ©1996 American Institute of Physics.