High density magnetic recording medium

Claim 1

A high density magnetic recording medium comprising a polyester film formed of a plurality of layers laminated by coextrusion and oriented by stretching, said plurality of layers including a first outer layer forming a rougher surface side of said polyester film and a second outer layer forming a less rough surface on an opposite side of said polyester film, said polyester film satisfying the conditions of the following relations (1) to (4); a back coat layer which is provided on the rougher surface side of said polyester film layer and whose surface roughness simultaneously satisfies the conditions of the following relations (5) and (6); and a magnetic film layer provided on the less rough surface of said polyester film; the following relations being as follows: Ra.sup.A ≦0.008 (1) 0.005≦Ra.sup.B ≦0.02 (2) 0.005≦Ra.sup.B -Ra.sup.A ≦0.015 (3) 9≦F.sub.5 ≦30 (4) 0.015≦Ra.sup.C ≦0.040 (5) 0.003≦Ra.sup.C -Ra.sup.B ≦0.035 (6) wherein Ra A stands for the average centerline roughness (μm) of the less rough surface side of the polyester film; Ra B stands for the average centerline roughness (μm) of the rougher surface of the polyester film; Ra C stands for the average centerline roughness (μm) of the surface of the back coat layer; and F 5 stands for the tensile strength in kilograms per square millimeter at 5% elongation in the longitudinal direction of the polyester film.