PURPOSE: To provide the titled compsn. which does not cause scattering of dust of a vulcanizing agent and the deposition thereof on a mixer and makes it possible to improve the working atmosphere and to greatly improve the productivity, by dissolving a specified resin vulcanization accelerator in a softener.

CONSTITUTION: A crosslinking agent compsn. for rubber is obtd. by dissolving a resin vulcanization accelerator composed of a reactive alkylformaldehyde resin (derivative) of the formula (wherein R is a 1W10C alkyl; R' is CH3, CH2Br; n is 1W5) having good adhesivity to metal, in a softener such as petroleum softener (e.g. paraffinic oil) or synthetic plasticizer (e.g. tricresyl phosphate). When mixed with rubber, the scattering of dust of a vulcanizing agent and the deposition thereof on a mixer are inhibited and it becomes possible to improve the working atmosphere and to conduct an internal mixing operation so that the mixing time can be reduced by 50% and the productivity can be greatly improved.