Provided are: a resin additive composition containing a specific metal phosphate, which composition has excellent dispersibility in a resin even when made into a pellet form and exhibits a desired physical property improving effect when incorporated into a resin; and a synthetic resin composition using the same. The resin additive composition contains, with respect to 100 parts by mass of (A) sodium-2,2′-methylene-bis(4,6-di-tert-butylphenyl)phosphate: 25 to 400 parts by mass of (B) a lithium phosphate compound represented by the following Formula (1); and 10 to 300 parts by mass of (C) a fatty acid metal salt represented by the following Formula (2). The component (C) is incorporated in an amount of 10 parts by mass to 50 parts by mass with respect to a total of 100 parts by mass of the components (A) and (B).