The stability of the light from arc plasma becomes a serious problem when we perform a various automatic control of arc welding on utilizing the image obtained by the light of arc. The fluctuating properties of the light are investigated on CO2 arc and their dependence on arc generated condition, their space distribution characteristics, the power density function and the amplitude density function of the fluctuating component are measured.The results are as follows;1) The new formula of power spectrum density function is introduced from the experimental results. This formula is decided by three parameters.2) Power of fluctuating component is obtained from the above mentioned experimental formula. It increases with arc current.3) The electric signal, into which the light of arc is converted, can be approximately regarded as stationary gaussian random process with constant component judging from the results of the measurement of amplitude density function.4) The space distribution characteristics of fluctuating and constant components depend on arc gene-rated condition and they show flat profile within the region of the obtained image if the relation of arc current and voltage is porper.