PURPOSE: To obtain an oil immersed electrical machine using a packing of nitrile rubber having freezing resistance enduring low temperatures below -20°C, heat resistance enduring high temperatures above 110°C and oil resistance.

CONSTITUTION: A rubber packing made of rubber composed by mixing nitrile rubber, 100 part by weight, having acrylonitrile unit 32±2% by weight with dioctyl sebacate 10±2 part by weight, and carbon powder 75±5 part by weight and insulating oil having a natural resistance value of 1×1013Ω.cm or more by JIS C2101 test method (method revised in 1978) are used. The rubber material mixed nitrile rubber, dioctyl sebacate, and carbon powder within said specific ranges is excellent in heat resistance, oil resistance, non-oily contamination proof in addition to freezing resistance.