Suitability for sizing is effectively imparted to paper while reducing the amounts of an internal sizing agent and aluminum sulfate to be used. An additive for papermaking is provided which is a mixture (pretreated filler) of a filler and either a cationic copolymer having hydrophobic groups having a degree of quaternization of 40 mol% or higher or an amphipathic copolymer having hydrophobic groups which have the same degree of quaternization and in which the proportion of the anion equivalent to the cation equivalent is 0.1-90%. This pretreated filler is one comprising a filler having moderate water repellency imparted thereto. When this pretreated filler is added to a pulp slurry and paper is formed therefrom by a wet process, the pretreated filler can be efficiently adsorbed onto pulp fibers anionically charged. Thus, suitability for sizing can be effectively imparted to the paper, while reducing the amounts of an internal sizing agent and aluminum sulfate to be used. A reduction in sizing agent amount can hence be attained.