Polymerizable composition and hydrophilizing treatment method

A polymerizable composition comprising:

• a polymerizable compound (A);
• a polymerization initiator (B);
• inorganic fine particles (C);
• and
• a solvent(S),
• wherein the polymerizable compound (A) comprises a polymerizable betaine compound (A1) and an adhesive polymerizable compound (A2),
• the polymerizable betaine compound (A1) has an ethylenic unsaturated double bond and a betaine structure,
• the adhesive polymerizable compound (A2) has an ethylenic unsaturated double bond and at least one adhesive group selected from the group consisting of a hydrolysable silyl group, −NH2, a carboxy group, a cyano group and a hydroxyl group,
• a ratio of a mass of the adhesive polymerizable compound (A2) with respect to a mass of the polymerizable compound (A) is 0.1% by mass or more and 5% by mass or less,
• the polymerization initiator (B) comprises a water-soluble radical polymerization initiator (B1),
• the inorganic fine particles (C) have an average particle diameter of 5 nm or more and 5000 nm or less based on a specific surface area of the inorganic fine particles (C) obtained by BET method,
• the inorganic fine particles (C) have a functional group capable of forming a covalent bond with a polymer of the polymerizable compound (A), and
• the solvent(S) is an aqueous solution of alcohol.