PURPOSE: To provide the titled compsn. which has excellent vulcanization properties and scouring processability and in which red phosphorus is well-dispersed in a rubber component, by blending a pasty mixture consisting of red phosphorus and a liquid rubber with a raw rubber.

CONSTITUTION: 2.5W7.5pts.wt. (in terms of red phosphorus) pasty mixture consisting of 40W50wt% red phosphorus and 60W50wt% liquid rubber (e.g. liquid polyisoprene rubber having an MW of 27,000W50,000) is blended with 100pts.wt. raw rubber such as polyisoprene rubber.