To provide a film for transfer medium substrates which does not cause blocking and has handling properties such as a rolling property kept in a high level even if a smoothing of the transfer medium substrates is promoted to improve a glossiness of an overcoat layer transferred using the transfer medium.

The polyester film for transfer medium substrates has a film layer of a microscopic uneven structure at least on one side of the biaxially-stretched polyester film, and the film layer has 104-108/mm2 inert particles having an average particle diameter of 8-50 nm, "a" due to AFM of the outer surface (surface A) of the film layer is 1-10 nm, and the surface is a surface to laminate ink.