This paper proposed the method as an estimation on the size of discharge spots through observation on traces after the discharge arose in circumstances gases mixed hydrocarbon gas. Namely, the circular carbonaceous deposit and the carbonaceous heap are observed on cathode and anode surface, respectively, after the short gap discharge arises in N2+NO+CH4 gases. The current density, which is the normal conversion current density, is calculated from the size of the trace of discharge and its value is about 1.010-9 A/(cm2 Pa2) in case that the concentration of CH4 is 0.6%. The value is about 1/5 of values that are reported in the former articles and is reasonable one.