A magnetic recording medium used for the thermally assisted magnetic recording system which fires a laser beam at a magnetic recording medium to partially heat the medium and applies a magnetic field from the outside to the part heated to lower the coercivity for recording. The magnetic recording medium is configured by a glass substrate on which a heat radiation layer, heat retention layer, intermediate layer, and recording layer are stacked. Further, the heat retention layer is configured by a member having an effective refractive index lower than the effective refractive index of the recording layer and having an temperature diffusion coefficient determined by the specific heat, density, and heat conductivity rate higher than glass and lower than metal. The material with a high temperature diffusion coefficient is used lowered in temperature diffusion coefficient using a porous structure or granular structure.