To obtain the subject agent capable of inexpensively and readily imparting both anti-microorganismic property and waterproof property to a material to be treated with excellent continuity by combinedly using a specific alkoxysilane with a specific quaternary ammonium salt.

This waterproof agent combinedly contains an alkoxysilane of formula I (R1 is a 1-4C alkyl; (n) is 1 or 2) or its partially hydrolyzed condensation product and a quaternary ammonium salt of formula II (R2 and R4 are each an alkyl; R3 is an alkyl; Xm- is a halogen ion, a sulfuric acid ion or a sulfurous acid ion; (m) is 1 or 2; R5 is an alkylene, etc.; R6 is an alkyl) or its partially hydrolyzed condensate. Preferably, a partially hydrolyzed condensation product of a trialkoxysilane expressed by formula I wherein (n) is 1 and R1 is a 1-3C alkyl and a quaternary ammonium salt expressed by formula II wherein Xm- is a halogen ion are used. The waterproof agent is penetrated into a material to be treated for 1min-24hrs, and in the process, concentration of the compound of the formula II in the last 1/10-1/2hr of the whole penetrating time is made higher than that before this time.