ilica, 0.5???6 parts of a silane coupling agent represented by the general formula of (OR).sub. 3SiCn H.sub. 2nSm Cn H.sub. 2nSi(OR).sub. 3 or (OR)3 SiCn H2 nX (wherein R is an alkyl group, X is a mercapto group, an amino group or an epoxy group, and n and m are a positive integer, respectively) and a given amount of a softening agent based on rubber ingredient composed of natural rubber and styre