A water-based surface treatment agent includes an organic-inorganic silicon compound (A) and inorganic particle (B), wherein the organic-inorganic silicon compound (A) is adapted to be a compound that is obtained by mixing a colloidal silica (C) and an organoalkoxysilane (D). In this regard, the ratio (MB/MA) between the solid content mass MA of the organic-inorganic silicon compound (A) and the solid content mass MB of the inorganic particle (B) preferably falls within the range from 0.2 or more and 2.0 or less, the average particle size of the organic-inorganic silicon compound (A) preferably falls within the range of 3 nm or more and 500 nm or less. In addition, the average particle size of the inorganic particle (B) preferably falls within the range of 10 nm or more and 600 nm or less.