A composite multi-layer substrate comprising: a flat plate-like core member (21, 41) formed of a material having electric conductivity, heat conductivity and rigidity; a front side resin layer (22, 42) covering a front surface of said core member(21, 41); a rear side resin layer (23, 43) covering a rear surface of said core member; a bottomless hole (24, 44) formed in said core member (21, 41) penetrating said front and rear of said core member (21, 41), and an electronic component (25, 48) mounted in said bottomless hole (24, 44), the thickness (Hc) of the electronic component (25, 48) after mounting in said bottomless hole (24, 44) is thinner than the thickness (Ha) of said core member (21, 41); characterised in that a first groove (110, 111) which is formed in either one or both of said front side resin layer (22, 42) and said rear side resin layer (23, 43), is located directly above or directly below said electronic component (25, 48), and extends in a vertical direction of the front surface and the rear surface of said core member (21, 41); a first electrode (63, 64) is formed on an interior portion of said first groove (110, 111); a second groove (35, 36) which is formed in either one or both of said front side resin layer (22, 42) and said rear side resin layer (23, 43), is located in areas directly above or directly below said core member (21, 41) excluding areas directly above and directly below said electronic component (25, 48), and extends in the vertical direction of the front surface and the rear surface of said core member (21, 41); a second electrode (37, 38) is formed on an interior portion of said second groove (35, 36), wherein an electrical connection to said electronic component (25, 48) is provided via said first electrode (63, 64), and an electrical connection to said core member (21, 41) is provided via said second groove (35, 36), and wherein said electronic component (25, 48) is structured such that, in a state where said electronic component (25, 48) is mounted in said bottomless hole (24, 44) with reference to a position of a surface in which an opening of said bottomless hole (24, 44) is formed, a position of at least one surface facing the opening among two surfaces of said electronic component (25, 48) is embedded within an interior portion of said bottomless hole (24, 44). a flat plate-like core member (21, 41) formed of a material having electric conductivity, heat conductivity and rigidity; a front side resin layer (22, 42) covering a front surface of said core member(21, 41); a rear side resin layer (23, 43) covering a rear surface of said core member; a bottomless hole (24, 44) formed in said core member (21, 41) penetrating said front and rear of said core member (21, 41), and an electronic component (25, 48) mounted in said bottomless hole (24, 44), the thickness (Hc) of the electronic component (25, 48) after mounting in said bottomless hole (24, 44) is thinner than the thickness (Ha) of said core member (21, 41); characterised in that a first groove (110, 111) which is formed in either one or both of said front side resin layer (22, 42) and said rear side resin layer (23, 43), is located directly above or directly below said electronic component (25, 48), and extends in a vertical direction of the front surface and the rear surface of said core member (21, 41); a first electrode (63, 64) is formed on an interior portion of said first groove (110, 111); a second groove (35, 36) which is formed in either one or both of said front side resin layer (22, 42) and said rear side resin layer (23, 43), is located in areas directly above or directly below said core member (21, 41) excluding areas directly above and directly below said electronic component (25, 48), and extends in the vertical direction of the front surface and the rear surface of said core member (21, 41); a second electrode (37, 38) is formed on an interior portion of said second groove (35, 36), wherein an electrical connection to said electronic component (25, 48) is provided via said first electrode (63, 64), and an electrical connection to said core member (21, 41) is provided via said second groove (35, 36), and wherein said electronic component (25, 48) is structured such that, in a state where said electronic component (25, 48) is mounted in said bottomless hole (24, 44) with reference to a position of a surface in which an opening of said bottomless hole (24, 44) is formed, a position of at least one surface facing the opening among two surfaces of said electronic component (25, 48) is embedded within an interior portion of said bottomless hole (24, 44).