Release paper base paper, includes; a substrate; and a coating agent containing a polyvinyl alcohol (A) and a compound (B), the substrate coated with the coating agent, wherein the compound (B) is unsaturated carboxylic acid or a derivative thereof having a pKa of less than 4.00, and 0.1 part by mass or more and 20 parts by mass or less of the compound (B) is contained based on 100 parts by mass of the polyvinyl alcohol (A). The release paper base paper of the present invention is excellent in sealing properties for silicone and waterproofness. The release paper base paper of the present invention is also capable of accelerating curing of silicone in a release layer.