Bonded substrate, metal circuit board, and circuit board

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 an edge of the first surface, which is located outside an 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 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 a peripheral edge of the first surface to a portion corresponding to a peripheral edge of the second surface in an orthogonal direction orthogonal to the stacking direction is defined as a peripheral surface length A (mm), a length of a protruding portion of the bonding member in the orthogonal direction is defined as a protrusion length B (mm), and a distance from the first surface to the second surface is defined as a thickness C (mm) of the metal plate, the following first expression and the following second expression are satisfied.
• 0.032≤B/(A+B)≤0.400 (First Expression)
• 0.5 (mm)≤C≤2.0 (mm) (Second Expression)