PURPOSE: To stabilize the titled compound useful as dry analytical elements, photosensitive materials, etc. at a high level without substantial diffusivity nor volatility, by bringing a diazonium salt into contact with a specific cured sulfonic acid resin.

CONSTITUTION: A diazonium salt is brought into contact with a cured sulfonic acid resin, prepared by crosslinking an addition polymer having ≥10mol%, preferably ≥70mol% groups derived from 2-acrylamide-2-methylpropanesulfonic acid and/or 2-methacryl-2-methylpropanesulfonic acid as constituent repeating units with an epoxy compound and stabilized. The diazonium salt is preferably in the state of a composition prepared by incorporating with the cured sulfonic acid resin. The content of the above-mentioned diazonium salt in the above- mentioned composition is ≤50wt%, preferably ≤30wt%.