A method of manufacturing an insulating circuit board with a heatsink including an insulating circuit board and a heatsink, the insulating circuit board including a circuit layer that is formed on a first surface of an insulating layer and a metal layer that is formed on a second surface of the insulating layer, the heatsink being bonded to the metal layer side of the insulating circuit board, the metal layer being formed of aluminum and having a configuration in which an indentation hardness of a thickness center portion is lower than 50 mgf/μm 2, a bonding surface of the heatsink with the insulating circuit board being formed of an aluminum alloy having a solidus temperature of 650°C or lower, and the method comprising: a high alloy element concentration portion forming step of forming a high alloy element concentration portion having a higher alloy element concentration than the thickness center portion of the metal layer and having a solidus temperature of 650°C or lower by laminating a clad material including a core material and a brazing material layer formed on both surfaces of the core material on a region of the metal layer opposite to the insulating layer to obtain a laminate and heating the laminate to diffuse an alloy element of the brazing material layer; and a heatsink bonding step of bonding the heatsink to the metal layer by laminating a copper bonding material formed of copper or a copper alloy between the metal layer and a bonding surface of the heatsink and bonding the metal layer and the copper bonding material to each other and bonding the copper bonding material and the heatsink to each other by solid phase diffusion bonding, wherein a ratio tb/ta of a thickness tb of the brazing material layer to a thickness ta of the core material in the clad material is in a range of 0.1 to 0.3. the insulating circuit board including a circuit layer that is formed on a first surface of an insulating layer and a metal layer that is formed on a second surface of the insulating layer, the heatsink being bonded to the metal layer side of the insulating circuit board, the metal layer being formed of aluminum and having a configuration in which an indentation hardness of a thickness center portion is lower than 50 mgf/μm 2, a bonding surface of the heatsink with the insulating circuit board being formed of an aluminum alloy having a solidus temperature of 650°C or lower, and the method comprising: a high alloy element concentration portion forming step of forming a high alloy element concentration portion having a higher alloy element concentration than the thickness center portion of the metal layer and having a solidus temperature of 650°C or lower by laminating a clad material including a core material and a brazing material layer formed on both surfaces of the core material on a region of the metal layer opposite to the insulating layer to obtain a laminate and heating the laminate to diffuse an alloy element of the brazing material layer; and a heatsink bonding step of bonding the heatsink to the metal layer by laminating a copper bonding material formed of copper or a copper alloy between the metal layer and a bonding surface of the heatsink and bonding the metal layer and the copper bonding material to each other and bonding the copper bonding material and the heatsink to each other by solid phase diffusion bonding, wherein a ratio tb/ta of a thickness tb of the brazing material layer to a thickness ta of the core material in the clad material is in a range of 0.1 to 0.3. a high alloy element concentration portion forming step of forming a high alloy element concentration portion having a higher alloy element concentration than the thickness center portion of the metal layer and having a solidus temperature of 650°C or lower by laminating a clad material including a core material and a brazing material layer formed on both surfaces of the core material on a region of the metal layer opposite to the insulating layer to obtain a laminate and heating the laminate to diffuse an alloy element of the brazing material layer; and a heatsink bonding step of bonding the heatsink to the metal layer by laminating a copper bonding material formed of copper or a copper alloy between the metal layer and a bonding surface of the heatsink and bonding the metal layer and the copper bonding material to each other and bonding the copper bonding material and the heatsink to each other by solid phase diffusion bonding, wherein a ratio tb/ta of a thickness tb of the brazing material layer to a thickness ta of the core material in the clad material is in a range of 0.1 to 0.3.