A heat sink substrate has a composite structure including a three-dimensional network structure of SiC ceramic having pores infiltrated with Si, and has a thermal conductivity of not less than 150 W/m·K and an oxygen content of not greater than 7 ppm. The heat sink substrate is easily allowed to have an increased surface area. Further, the heat sink substrate has a higher thermal conductivity and a coefficient thermal expansion close to that of the SiC. Therefore, the heat sink substrate is superior in the efficiency of heat conduction from a semiconductor device. The heat sink substrate is produced by infiltrating a thermally melted Si into the pores of the three-dimensional network structure in a non-oxidative atmosphere in the presence of an oxygen absorber.