PURPOSE: To provide a fungicidal mildew-proofing agent for wood composed mainly of a specific alkyl ammonium compound anion containing a residue of an acid having polymerizable double bond and made to be resistant to desolvation by polymerizing in a state applied to or impregnated in wood.

CONSTITUTION: The objective fungicidal mildew-proofing agent for wood contains the compound of formula (R1 is 8-22C alkyl or alkenyl; R2 and R3 are 1-12C alkyl; R4 is 2-4C alkylene; n is 1-5; m is 1 or 2; X is residue of an acid having polymerizable vinyl group) as a main component. The compound of formula may be used singly or as a combination of two or more compounds or after mixing with a proper solvent or a polymerizable monomer having vinyl group. The compound is preferably used in combination with a polymerization initiator. Wood is imparted with durable excellent fungicidal mildew- proofing effect by applying or impregnating the fungicidal mildew-proofing agent in wood and polymerizing the compound with ultraviolet irradiation or heat-treatment.