The present invention provides a solubilizing agent that can give solubility or dispersibility in resins to copper-containing phosphorus compounds, and a composition containing the solubilizing agent. The solubilizing agent of the invention comprises a compound represented by the following general formula (100) having a main chain with an oxyalkylene unit. [wherein R111 represents hydrogen or a b-valent organic group, R112 represents C2-4 alkylene, R113 represents hydrogen, alkyl, aryl or acyl, a represents an integer of 1-50 and b represents an integer of 1-4.]