Abstract: Provided are a biaxially oriented base film formed from polyethylene-2,6-naphthalenedicarboxylate as a raw material and a high-density magnetic recording floppy disk using the same. The base film and the floppy disk are characteristic in that they inhibit vibration in the film surface and have excellent viscoelasticity thereby to accomplish a contact with a magnetic head under given conditions even when rotated at high speed.

Abstract: A biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film, which can be used as a base film for a recording medium capable of high-density recording, characterized in that when h (unit: nm) stands for a height of a protrusion formed on the film surface, the numbers of protrusions in the range of 1.ltoreq.h<50, 50.ltoreq.h<100, and 100.ltoreq.h fall each within a specified range, that the film has a plane orientation coefficient (NS) and an average refractive index (n.sub.a) which satisfy the relationship NS.gtoreq.1.607n.sub.a -2.434, that the film has a surface roughness (Ra) of 2-10 nm and that the film has a heat shrinkage in the longitudinal direction of 0.08% or less when heat-treated under no load at 70.degree. C. for 1 hour.