$SF_6$ is widely used as gas insulation medium because of having excellent dielectric and arc-quenching properties. However the use of it is getting to be suppressed from the viewpoint of mitigating global warming. For the development of environmentally-benign electric power equipment and system, novel gases or mixture gases are strongly required as the substitute of $SF_6$ gas. In this study the authors constructed an experimental system to investigate insulation properties of the mixed gases composed of negative $gas(SF_6)$ and electron deceleration gases$(N_2;and;CO_2)$. Breakdown and prebreakdown characteristics of $SF_6/N_2/CO_2$ mixture gases were investigated using the above mentioned system for different rates and gas pressures.????????????6???????