A bonded substrate comprising: a substrate; a metal plate that forms a stacked state along with the substrate, has a first surface on the substrate side and a second surface on an opposite side of the first surface, and has a peripheral edge of the first surface, which is located outside a peripheral edge of the second surface when viewed in a stacking direction; and a bonding member that is disposed between the substrate and the metal plate, so as to bond the metal plate to the substrate, and protrudes from the peripheral edge over an entire periphery of the metal plate when viewed from the stacking direction, wherein in a case where, in at least one of a plurality of cut surfaces obtained by cutting the bonded substrate along the stacking direction, a length from a portion corresponding to the peripheral edge of the first surface to a portion corresponding to the peripheral edge of the second surface in an orthogonal direction (x) orthogonal to the stacking direction is defined as a peripheral surface length A, a length of a protruding portion of the bonding member in the orthogonal direction (x) is defined as a protrusion length B, and a distance from the first surface to the second surface is defined as a thickness C of the metal plate, the following first expression and the following second expression are satisfied, a peripheral surface of the metal plate, which connects the entire peripheral edge of the first surface and the entire peripheral edge of the second surface is a surface including an inclined surface, in at least one of the plurality of cut surfaces obtained by cutting the bonded substrate along the stacking direction, an inclination angle θ of a portion corresponding to the inclined surface with respect to the orthogonal direction (x) satisfies the following fourth expression, a ratio of an area S2 and an area S1 satisfies the following third expression, wherein the area of a cross section corresponding to a right-angled isosceles triangle of which a hypotenuse is a linear line connecting the peripheral edge and a point at which the drawn linear line intersects with the upper surface is set as S1; and the area of a cross section of a protrusion from the linear line connecting the peripheral edge and the point at which the drawn linear line intersects with the upper surface toward an outside of the ceramic substrate is set as S2; and wherein the protrusion length B is any length in a range of being equal to or more than 10×10 -3 mm and equal to or less than 100×10 -3 mm. 0.032 ≤ B / A + B ≤ 0.400 0.5 mm ≤ C ≤ 2.0 mm 0.6 ≤ S 2 / S 1 θ ≥ 69. a substrate; a metal plate that forms a stacked state along with the substrate, has a first surface on the substrate side and a second surface on an opposite side of the first surface, and has a peripheral edge of the first surface, which is located outside a peripheral edge of the second surface when viewed in a stacking direction; and a bonding member that is disposed between the substrate and the metal plate, so as to bond the metal plate to the substrate, and protrudes from the peripheral edge over an entire periphery of the metal plate when viewed from the stacking direction, wherein in a case where, in at least one of a plurality of cut surfaces obtained by cutting the bonded substrate along the stacking direction, a length from a portion corresponding to the peripheral edge of the first surface to a portion corresponding to the peripheral edge of the second surface in an orthogonal direction (x) orthogonal to the stacking direction is defined as a peripheral surface length A, a length of a protruding portion of the bonding member in the orthogonal direction (x) is defined as a protrusion length B, and a distance from the first surface to the second surface is defined as a thickness C of the metal plate, the following first expression and the following second expression are satisfied, a peripheral surface of the metal plate, which connects the entire peripheral edge of the first surface and the entire peripheral edge of the second surface is a surface including an inclined surface, in at least one of the plurality of cut surfaces obtained by cutting the bonded substrate along the stacking direction, an inclination angle θ of a portion corresponding to the inclined surface with respect to the orthogonal direction (x) satisfies the following fourth expression, a ratio of an area S2 and an area S1 satisfies the following third expression, wherein the area of a cross section corresponding to a right-angled isosceles triangle of which a hypotenuse is a linear line connecting the peripheral edge and a point at which the drawn linear line intersects with the upper surface is set as S1; and the area of a cross section of a protrusion from the linear line connecting the peripheral edge and the point at which the drawn linear line intersects with the upper surface toward an outside of the ceramic substrate is set as S2; and wherein the protrusion length B is any length in a range of being equal to or more than 10×10 -3 mm and equal to or less than 100×10 -3 mm. 0.032 ≤ B / A + B ≤ 0.400 0.5 mm ≤ C ≤ 2.0 mm 0.6 ≤ S 2 / S 1 θ ≥ 69.