Surface treatment method for powder, surface treatment agent composition used therein, and surface-treated powder

The present invention provides a powder surface treatment method comprising (i) a step for mixing 0.001-20 parts by mass of a surface treatment agent composition with 100 parts by mass of a powder, and (ii) a step for adding and mixing an acidic solution, the surface treatment agent composition contains the following (a) to (d): (a) one or more substances selected from the group consisting of N-acylamino acids having a carbon chain length of C8-C22 and salts thereof; (b) one or more substances selected from the group consisting of monohydric alcohols and polyhydric alcohols; (c) water; (d) one or more substances selected from the group consisting of the following (d1) amino acids having an isoelectric point of 7.5 to 11 and (d2) amino acids having an isoelectric point of 7.5 to 11; and (d2) a compound that generates an ion selected from the group consisting of an aluminum ion, a magnesium ion, a calcium ion, a zinc ion, a zirconium oxide ion, and a titanium ion in the surface treatment agent composition. 本发明提