NOVELTY - A biaxially-oriented polyester film has a laminated structure formed by laminating two or more layers (B) containing inert particles formed on surface(s) of layer (A) containing polyester and non-crystalline thermoplastic resin. A layer (C) containing particles and water-soluble polymer, is formed on the surface of layer (A). Protrusion formed by aggregation of particles along direction with low Young's modulus, is formed on layer (C). The humidity coefficient of expansion of biaxially-oriented polyester film is 0-6 ppm/%RH.    USE - Biaxially-oriented polyester film is used for magnetic recording medium electric insulations, capacitor, circuit material and solar cell material.    ADVANTAGE - The biaxially-oriented polyester film has excellent heat resistance, dimensional stability, surface property, durability and rigidity, and reduced dimensional change. The magnetic recording medium using biaxially-oriented polyester film has excellent magnetic parametric characteristics and high density.    DETAILED DESCRIPTION - A biaxially-oriented polyester film has a laminated structure formed by laminating two or more layers (B) containing inert particles formed on surface(s) of layer (A) containing polyester and non-crystalline thermoplastic resin. The glass transition temperature of non-crystalline thermoplastic resin is 220-280 degrees C, which is higher than the glass transition temperature of polyester contained in layer (A). A layer (C) containing particles and water-soluble polymer, is formed on the surface of layer (A). Protrusion formed by aggregation of particles along direction with low Young's modulus, is formed on layer (C). The humidity coefficient of expansion of biaxially-oriented polyester film is 0-6 ppm/%RH. The protrusion interval of biaxially-oriented polyester film in direction with high Young's modulus is 0.3-3 mu m. The surface roughness of layer (A) is 0.1-5 nm. The surface roughness of layer (B) is 5-15 nm. The surface roughness of layer (B) is greater than or equal to that of surface roughens layer (A).