Provided is a chromium-free metal surface treatment agent excellent in rust inhibitive performance which can be used in a surface treatment of metal products, in particular, galvanized metal products. The metal surface treatment agent consists essentially of a siliceous binder solution comprising water and/or alcohol as a solvent, and the concentration of the silica component is 8 to 25% by weight. In particular, a metal surface treatment agent containing an alcoholic solution of alkoxysilane oligomer having a weight averaged molecular weight of 1000 to 10000, which is obtained by hydrolyzed condensation polymerization of alkoxysilane, can suppress the occurrence of white rusting for a long time. It is preferred that 3 to 25 parts by weight of a dispersed nano-sized titanium dioxide powder having an average primary particle size of not more than 70 nm, per 100 parts by weight of the silica component be suspended in the siliceous binder solution. The rust inhibitive performance of the metal surface treatment agent can be further improved by using an alcohol-soluble polyvinyl butyral resin and by adding a silane coupling agent having a vinyl or epoxy group as a functional group, in combination with a dispersed nano-sized powder of titanium dioxide.