A heat-conductive rubber composition material, which comprises a rubber essentially consisting of an acrylic rubber, a butyl rubber, an ethylene propylene rubber or a blended rubber which comprises at least two of acrylic rubber, butyl rubber, and ethylene propylene rubber are blended, and 50 to 85 weight % of hydrophobic magnesium oxide powder, a heat-conductive rubber sheet prepared by forming the above thermal conductive rubber composition material, and a heat-conductive sheet, which comprises a soft metal sheet and a rubber layer formed on at least one surface of the soft metal sheet.