To obtain the subject admixure acquired by mixing a silane coupling agent with a free radical generator, a silanol condensating catalyst and a specific aging preventive agent and dissolving them, and necessary for producing hydrocrosslinked molded products having excellent thermal aging resistance for a long period of time.

This admixute is obtained by mixing a silane coupling agent at least with a free radical generator, a silanol condensating catalyst and an aging preventive agent showing its residual percentage of ≥95 wt.% in the admixure as measured by liquid chromatography after heating 50°C × 10 days [pref. 4,4'-thiobis(3-methyl-6-t-butylphenol), polymerized 2,2,4-trimethyl-1,2- dihydroquinoline, 2,2'-methylene-bis(4-ethyl-6-t-butylphenol), diphenylamine derivatives, 4,4'-bis(α,α'-dimethylbenzyl)diphenylamine or the like] and dissolving them.