A circuit substrate comprising: a ceramic substrate including a first surface and a second surface, and having a thickness of 0.4 mm or less; a first metal plate bonded to the first surface via a first bonding layer, and having a thickness of 0.6 mm or more; and a second metal plate bonded to the second surface via a second bonding layer, and having a thickness of 0.6 mm or more, wherein a side surface of the first metal plate and an inner surface thereof define an angle of 40 degrees or more and 84 degrees or less therebetween, wherein the first bonding layer has a first portion protruding from the interface between the first surface and the first metal plate and extending on the first surface, wherein the second bonding layer has a second portion protruding from the interface between the second surface and the second metal plate and extending on the second surface, wherein at least one selected from the group consisting of the first and second bonding layers (5, 7) contains Ag, Cu and Ti, wherein a three-point bending strength of the ceramic substrate is 500 MPa or more, and wherein at least one selected from the group consisting of a ratio L1/H1 of a length L1 (μm) of the first portion to a thickness H1 (μm) thereof and a ratio L2/H2 of a length L2 (μm) of the second portion to a thickness H2 (μm) thereof is 0.5 or more and 3.0 or less, characterized in that at least one selected from the group consisting of an average value of first Vickers hardnesses of 10 places of the first portion and an average value of second Vickers hardnesses of 10 places of the second portion is 2452 MPa (250 kgf/mm 2) or less, at least one selected from the group consisting of a difference between a maximum value and a minimum value of the first Vickers hardnesses and a difference between a maximum value and a minimum value of the second Vickers hardnesses is 490 MPa (50 kgf/mm 2) or less, a mass ratio of Ag to Cu in each of the first and second bonding layers (5, 7) is 2.1 or less, and a bonding strength of each of the first and second metal plates (3, 4) is 17 kN/m or more. a ceramic substrate including a first surface and a second surface, and having a thickness of 0.4 mm or less; a first metal plate bonded to the first surface via a first bonding layer, and having a thickness of 0.6 mm or more; and a second metal plate bonded to the second surface via a second bonding layer, and having a thickness of 0.6 mm or more, wherein a side surface of the first metal plate and an inner surface thereof define an angle of 40 degrees or more and 84 degrees or less therebetween, wherein the first bonding layer has a first portion protruding from the interface between the first surface and the first metal plate and extending on the first surface, wherein the second bonding layer has a second portion protruding from the interface between the second surface and the second metal plate and extending on the second surface, wherein at least one selected from the group consisting of the first and second bonding layers (5, 7) contains Ag, Cu and Ti, wherein a three-point bending strength of the ceramic substrate is 500 MPa or more, and wherein at least one selected from the group consisting of a ratio L1/H1 of a length L1 (μm) of the first portion to a thickness H1 (μm) thereof and a ratio L2/H2 of a length L2 (μm) of the second portion to a thickness H2 (μm) thereof is 0.5 or more and 3.0 or less, characterized in that at least one selected from the group consisting of an average value of first Vickers hardnesses of 10 places of the first portion and an average value of second Vickers hardnesses of 10 places of the second portion is 2452 MPa (250 kgf/mm 2) or less, at least one selected from the group consisting of a difference between a maximum value and a minimum value of the first Vickers hardnesses and a difference between a maximum value and a minimum value of the second Vickers hardnesses is 490 MPa (50 kgf/mm 2) or less, a mass ratio of Ag to Cu in each of the first and second bonding layers (5, 7) is 2.1 or less, and a bonding strength of each of the first and second metal plates (3, 4) is 17 kN/m or more.