the present invention is one that is obtained by polymerizing a mixture of c 5 c 9 olefins the addition of the petroleum resin contributes to a significant enhancement of the maximum value of loss tangent but the degree of its effectiveness will vary considerably depending on its composition and molecular weight stated more specifically a preferred petroleum resin is one that contains at least 50 wt of indene and styrene which are c 9 components more desirably styrene is present in a larger amount than indene it is also preferred that the petroleum resin has a number average molecular weight of 500 1 500 and outside this range the loss tangent of the resin composition will decrease the petroleum resin is preferably added in an amount of 3 200 parts by weight more preferably from 10 to 100 parts by weight per 100 parts by weight of the polyvinyl chloride based resin if the content of the petroleum resin is less than 3 parts by weight it does not make much contribution to the increase in loss tangent on the other hand if the petroleum resin is added in an amount exceeding 200 parts by weight the processability of the resin composition will deteriorate markedly the petroleum resin is certainly effective in increasing the loss tangent of the resin composition but its effectiveness will vary greatly depending upon the type of the plasticizer which is added as the third component in other words particularly great advantages will be attained if the plasticizer which is to be added as the.