An insulating thermally conductive resin composition, comprising: a phase-separated structure including: a first resin phase in which a first resin continues three-dimensionally; and a second resin phase different from the first resin phase and formed of a second resin; small-diameter inorganic filler unevenly distributed in the first resin phase; and large-diameter inorganic filler that spans the first resin phase and the second resin phase and thermally connects pieces of the small-diameter inorganic filler to one another, the small-diameter inorganic filler being unevenly distributed in the first resin phase, wherein an average particle diameter of the small-diameter inorganic filler is 0.1 to 15 μm, and an average particle diameter of the large-diameter inorganic filler is larger than the average particle diameter of the small-diameter inorganic filler, and is 1 to 100 μm, the average particle diameters of the small-diameter inorganic filler and the large-diameter inorganic filler are measured by firing the insulating thermally conductive resin composition and isolating the small-diameter inorganic filler and the large-diameter inorganic filler from each other, and are measured by using a laser diffraction-type particle size distribution measuring instrument, wherein the first resin phase is formed of either one of a thermosetting resin and a thermoplastic resin, and the second resin phase is formed of other of the thermosetting resin and the thermoplastic resin, wherein the thermosetting resin is an epoxy resin, an unsaturated polyester resin, a vinyl ester resin, a phenol resin, a urethane resin, a urea resin, a melamine resin, a maleimide resin, a cyanate ester resin, an alkyd resin, an addition-curable polyimide resin, or combinations thereof, and the thermoplastic resin is a polyolefin-based resin, a polyamide-based resin, an elastomer-based resin, an acrylic resin, a polyester-based resin, engineering plastics, polyethylene, polypropylene, a Nylon resin, an acrylonitrile-butadiene-styrene resin, an ethylene acrylate resin, an ethylene vinyl acetate resin, and a polystyrene resin, a polyphenylene sulfide resin, a polycarbonate resin, a polyester elastomer resin, a polyamide elastomer resin, a liquid crystal polymer, a polybutylene terephthalate resin, or combinations thereof, wherein the small-diameter inorganic filler and the large-diameter inorganic filler are an inorganic compound containing at least one of boride, carbide, nitride, oxide, silicide, hydroxide, and carbonate, and wherein a ratio of the large-diameter inorganic filler in the sum of the small-diameter inorganic filler and the large-diameter inorganic filler is 5 to 60 % by volume. a phase-separated structure including: a first resin phase in which a first resin continues three-dimensionally; and a second resin phase different from the first resin phase and formed of a second resin; small-diameter inorganic filler unevenly distributed in the first resin phase; and large-diameter inorganic filler that spans the first resin phase and the second resin phase and thermally connects pieces of the small-diameter inorganic filler to one another, the small-diameter inorganic filler being unevenly distributed in the first resin phase, wherein an average particle diameter of the small-diameter inorganic filler is 0.1 to 15 μm, and an average particle diameter of the large-diameter inorganic filler is larger than the average particle diameter of the small-diameter inorganic filler, and is 1 to 100 μm, the average particle diameters of the small-diameter inorganic filler and the large-diameter inorganic filler are measured by firing the insulating thermally conductive resin composition and isolating the small-diameter inorganic filler and the large-diameter inorganic filler from each other, and are measured by using a laser diffraction-type particle size distribution measuring instrument, wherein the first resin phase is formed of either one of a thermosetting resin and a thermoplastic resin, and the second resin phase is formed of other of the thermosetting resin and the thermoplastic resin, wherein the thermosetting resin is an epoxy resin, an unsaturated polyester resin, a vinyl ester resin, a phenol resin, a urethane resin, a urea resin, a melamine resin, a maleimide resin, a cyanate ester resin, an alkyd resin, an addition-curable polyimide resin, or combinations thereof, and the thermoplastic resin is a polyolefin-based resin, a polyamide-based resin, an elastomer-based resin, an acrylic resin, a polyester-based resin, engineering plastics, polyethylene, polypropylene, a Nylon resin, an acrylonitrile-butadiene-styrene resin, an ethylene acrylate resin, an ethylene vinyl acetate resin, and a polystyrene resin, a polyphenylene sulfide resin, a polycarbonate resin, a polyester elastomer resin, a polyamide elastomer resin, a liquid crystal polymer, a polybutylene terephthalate resin, or combinations thereof, wherein the small-diameter inorganic filler and the large-diameter inorganic filler are an inorganic compound containing at least one of boride, carbide, nitride, oxide, silicide, hydroxide, and carbonate, and wherein a ratio of the large-diameter inorganic filler in the sum of the small-diameter inorganic filler and the large-diameter inorganic filler is 5 to 60 % by volume.