What is provided is a method of manufacturing an insulating circuit board with a heatsink including an insulating circuit board and a heatsink, the heatsink being bonded to the metal layer side of the insulating circuit board, the metal layer being formed of aluminum, and 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. This method includes a high alloy element concentration portion forming step (S02) of forming a high alloy element concentration portion and a heatsink bonding step (S03) of bonding the heatsink, in which a ratio tb/ta of a thickness tb of the brazing material layer to a thickness to of the core material in the clad material is in a range of 0.1 to 0.3.