A power module substrate (10, 110, 210, 310, 410, 1010) comprising: a ceramic substrate (11, 111, 211, 311, 411, 1011); a metal plate (22, 122, 222, 322, 422) which contains aluminum or an aluminum alloy, and which is stacked and bonded on a surface of the ceramic substrate (11, 111, 211, 311, 411, 1011); a concentration gradient layer (33, 133, 233) in which a concentration of one or more additional elements selected from Ag, Zn, Ge, and Mg gradually decreases with moving away from the bonding interface (30, 130, 230, 330) between the ceramic substrate (11, 111, 211, 311, 411, 1011) and the metal plate (22, 122, 222, 322, 422) in a stacked direction; and a high concentration part (32, 132, 232) of the one or more additional elements selected from Ag, Zn, Ge, and Mg formed in the bonding interface (30, 130, 230, 330), wherein the one or more additional elements selected from Ag, Zn, Ge, and Mg are solid-solubilized in said metal plate (22, 122, 222, 322, 422), wherein the Ag concentration in said metal plate (22, 122, 222, 322, 422) at a position 50 μm from the bonding interface (30, 130, 230, 330) with said ceramic substrate (11, 111, 211, 311, 411, 1011) is greater than or equal to 0.05% by mass and less than or equal to 10% by mass, or the total concentration of Zn, Ge, and Mg in said metal plate (22, 122, 222, 322, 422) at a position 50 μm from the bonding interface (30, 130, 230, 330) with said ceramic substrate (11, 111, 211, 311, 411, 1011) is greater than or equal to 0.01% by mass and less than or equal to 5% by mass, wherein the high concentration part (32, 132, 232) has a concentration of the one or more additional elements selected from Ag, Zn, Ge, and Mg that is two or more times higher than the concentration of the one or more additional elements in the metal plate (22, 122, 222, 322, 422), wherein the thickness of the high concentration part (32, 132, 232) is less than or equal to 4 nm, and wherein the concentration of the additional elements is an average value of measurements made at five points located at 50 μm from the bonding interface (30, 130, 230, 330) by means of EPMA analysis. a ceramic substrate (11, 111, 211, 311, 411, 1011); a metal plate (22, 122, 222, 322, 422) which contains aluminum or an aluminum alloy, and which is stacked and bonded on a surface of the ceramic substrate (11, 111, 211, 311, 411, 1011); a concentration gradient layer (33, 133, 233) in which a concentration of one or more additional elements selected from Ag, Zn, Ge, and Mg gradually decreases with moving away from the bonding interface (30, 130, 230, 330) between the ceramic substrate (11, 111, 211, 311, 411, 1011) and the metal plate (22, 122, 222, 322, 422) in a stacked direction; and a high concentration part (32, 132, 232) of the one or more additional elements selected from Ag, Zn, Ge, and Mg formed in the bonding interface (30, 130, 230, 330), wherein the one or more additional elements selected from Ag, Zn, Ge, and Mg are solid-solubilized in said metal plate (22, 122, 222, 322, 422), wherein the Ag concentration in said metal plate (22, 122, 222, 322, 422) at a position 50 μm from the bonding interface (30, 130, 230, 330) with said ceramic substrate (11, 111, 211, 311, 411, 1011) is greater than or equal to 0.05% by mass and less than or equal to 10% by mass, or the total concentration of Zn, Ge, and Mg in said metal plate (22, 122, 222, 322, 422) at a position 50 μm from the bonding interface (30, 130, 230, 330) with said ceramic substrate (11, 111, 211, 311, 411, 1011) is greater than or equal to 0.01% by mass and less than or equal to 5% by mass, wherein the high concentration part (32, 132, 232) has a concentration of the one or more additional elements selected from Ag, Zn, Ge, and Mg that is two or more times higher than the concentration of the one or more additional elements in the metal plate (22, 122, 222, 322, 422), wherein the thickness of the high concentration part (32, 132, 232) is less than or equal to 4 nm, and wherein the concentration of the additional elements is an average value of measurements made at five points located at 50 μm from the bonding interface (30, 130, 230, 330) by means of EPMA analysis.