A heat-diffusion sheet when a needle electrode having a cone having a height of 3 mm and a bottom surface diameter of 0.75 mm at a distal end section, wherein a 2.0-kV current voltage having a frequency of 60 Hz, is penetrated stepwise every 10 μm but also retained for 60 seconds before the penetration, a distance between the distal end of the needle electrode and an aluminum plate at the time of dielectric breakdown of the heat-diffusion sheet is larger than 0 μm and 80 μm or less, or the needle electrode short-circuits the aluminum plate without dielectric breakdown of the heat-diffusion sheet. The method includes a step of pre-heating a composition for heat-diffusion sheet at a pre-heating temperature lower than a curing starting temperature; and a curing step of heating the composition for heat-diffusion sheet at a temperature of the curing starting temperature or higher while pressurizing the pre-heated composition sheet for heat-diffusion sheet.