A method for producing an insulating circuit substrate with a heat sink including an insulating circuit substrate and a heat sink, the insulating circuit substrate 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 heat sink being bonded to the metal layer side of the insulating circuit substrate, the metal layer being formed of aluminum having a purity of at least 99 mass% or higher, an indentation hardness of the metal layer being lower than 490 MPa (50 mgf/μm 2), and a bonding surface of the heat sink with the insulating circuit substrate being formed of aluminum or an aluminum alloy having a solidus temperature of 650°C or lower, the method comprising: an aluminum bonding layer forming step of forming an aluminum bonding layer formed of aluminum or an aluminum alloy having a solidus temperature of 650°C or lower on a surface of the metal layer opposite to the insulating layer, and a heat sink bonding step of bonding the heat sink to the aluminum bonding layer by laminating a copper bonding material formed of copper or a copper alloy between the aluminum bonding layer and a bonding surface of the heat sink and bonding the aluminum bonding layer and the copper bonding material to each other and bonding the copper bonding material and the heat sink to each other by solid phase diffusion bonding, wherein in the measurement of the indentation hardness, a load-displacement relation is measured with a method according to ISO 14577 when a negative pressure was applied at a test load of 5000 mgf 49.03 N) using a triangular pyramidal diamond indenter called a Berkovich indenter having a dihedral angle of 114.8° to 115.1° and the indentation hardness was obtained from the expression Indentation Hardness=37.926×10 -3 (load [mgf]÷displacement [μm] 2). the insulating circuit substrate 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 heat sink being bonded to the metal layer side of the insulating circuit substrate, the metal layer being formed of aluminum having a purity of at least 99 mass% or higher, an indentation hardness of the metal layer being lower than 490 MPa (50 mgf/μm 2), and a bonding surface of the heat sink with the insulating circuit substrate being formed of aluminum or an aluminum alloy having a solidus temperature of 650°C or lower, the method comprising: an aluminum bonding layer forming step of forming an aluminum bonding layer formed of aluminum or an aluminum alloy having a solidus temperature of 650°C or lower on a surface of the metal layer opposite to the insulating layer, and a heat sink bonding step of bonding the heat sink to the aluminum bonding layer by laminating a copper bonding material formed of copper or a copper alloy between the aluminum bonding layer and a bonding surface of the heat sink and bonding the aluminum bonding layer and the copper bonding material to each other and bonding the copper bonding material and the heat sink to each other by solid phase diffusion bonding, wherein in the measurement of the indentation hardness, a load-displacement relation is measured with a method according to ISO 14577 when a negative pressure was applied at a test load of 5000 mgf 49.03 N) using a triangular pyramidal diamond indenter called a Berkovich indenter having a dihedral angle of 114.8° to 115.1° and the indentation hardness was obtained from the expression Indentation Hardness=37.926×10 -3 (load [mgf]÷displacement [μm] 2). an aluminum bonding layer forming step of forming an aluminum bonding layer formed of aluminum or an aluminum alloy having a solidus temperature of 650°C or lower on a surface of the metal layer opposite to the insulating layer, and a heat sink bonding step of bonding the heat sink to the aluminum bonding layer by laminating a copper bonding material formed of copper or a copper alloy between the aluminum bonding layer and a bonding surface of the heat sink and bonding the aluminum bonding layer and the copper bonding material to each other and bonding the copper bonding material and the heat sink to each other by solid phase diffusion bonding, wherein in the measurement of the indentation hardness, a load-displacement relation is measured with a method according to ISO 14577 when a negative pressure was applied at a test load of 5000 mgf 49.03 N) using a triangular pyramidal diamond indenter called a Berkovich indenter having a dihedral angle of 114.8° to 115.1° and the indentation hardness was obtained from the expression Indentation Hardness=37.926×10 -3 (load [mgf]÷displacement [μm] 2).