A method of producing a substrate, comprising: infiltrating, by squeeze casting at a pressure of at least 20 MPa, an aluminum alloy comprising 3 to 20 mass% of silicon into an isotropic graphite material to obtain an aluminum-graphite composite wherein at least 70% of the pores in the isotropic graphite material are infiltrated by the aluminium alloy, the isotropic graphite material being in the form of a rectangular parallelepiped with each edge having a length of 100 to 500 mm, having, as raw material, a coke-type graphite with a thermal conductivity of 100 to 200 W/mK at a temperature of 25 °C, a maximum/minimum ratio of 1 to 1.3 for thermal conductivity in three orthogonal directions, a coefficient of thermal expansion of 2 × 10 -6 to 5 × 10 -6 /K at a temperature of 25 °C to 150 °C, a maximum/minimum ratio of 1 to 1.3 for coefficient of thermal expansion in three orthogonal directions, and a porosity of 10 to 20 vol%; working the aluminum-graphite composite having a surface roughness (Ra) of 0.1 to 3, a thermal conductivity of 150 to 300 W/mK at a temperature of 25 °C, a maximum/minimum ratio of 1 to 1.3 for thermal conductivity in three orthogonal directions, a coefficient of thermal expansion of 4 × 10 -6 to 7.5 × 10 -6 /K at a temperature of 25 °C to 150 °C, a maximum/minimum ratio of 1 to 1.3 for coefficient of thermal expansion in three orthogonal directions, and a three-point bending strength of 50 to 150 MPa, using a multi-wire saw for forming plates of thickness 0.5 to 3 mm under the following conditions to: the abrasive grains bonded to the wires of the multi-wire saw being of at least one type chosen from among diamond, C-BN, silicon carbide and alumina, and having an average grain size of 10 to 100 μm; the wire diameter being 0.1 to 0.3 mm; the wire delivery rate being 100 to 700 m/min; and the cutting rate being 0.1 to 2 mm/min. infiltrating, by squeeze casting at a pressure of at least 20 MPa, an aluminum alloy comprising 3 to 20 mass% of silicon into an isotropic graphite material to obtain an aluminum-graphite composite wherein at least 70% of the pores in the isotropic graphite material are infiltrated by the aluminium alloy, the isotropic graphite material being in the form of a rectangular parallelepiped with each edge having a length of 100 to 500 mm, having, as raw material, a coke-type graphite with a thermal conductivity of 100 to 200 W/mK at a temperature of 25 °C, a maximum/minimum ratio of 1 to 1.3 for thermal conductivity in three orthogonal directions, a coefficient of thermal expansion of 2 × 10 -6 to 5 × 10 -6 /K at a temperature of 25 °C to 150 °C, a maximum/minimum ratio of 1 to 1.3 for coefficient of thermal expansion in three orthogonal directions, and a porosity of 10 to 20 vol%; working the aluminum-graphite composite having a surface roughness (Ra) of 0.1 to 3, a thermal conductivity of 150 to 300 W/mK at a temperature of 25 °C, a maximum/minimum ratio of 1 to 1.3 for thermal conductivity in three orthogonal directions, a coefficient of thermal expansion of 4 × 10 -6 to 7.5 × 10 -6 /K at a temperature of 25 °C to 150 °C, a maximum/minimum ratio of 1 to 1.3 for coefficient of thermal expansion in three orthogonal directions, and a three-point bending strength of 50 to 150 MPa, using a multi-wire saw for forming plates of thickness 0.5 to 3 mm under the following conditions to: the abrasive grains bonded to the wires of the multi-wire saw being of at least one type chosen from among diamond, C-BN, silicon carbide and alumina, and having an average grain size of 10 to 100 μm; the wire diameter being 0.1 to 0.3 mm; the wire delivery rate being 100 to 700 m/min; and the cutting rate being 0.1 to 2 mm/min. the abrasive grains bonded to the wires of the multi-wire saw being of at least one type chosen from among diamond, C-BN, silicon carbide and alumina, and having an average grain size of 10 to 100 μm; the wire diameter being 0.1 to 0.3 mm; the wire delivery rate being 100 to 700 m/min; and the cutting rate being 0.1 to 2 mm/min.