In a peeling starting portion preparing method of a laminate, for the laminate including a first substrate and a second substrate peelably attached via an adsorption layer, a knife is inserted with a predetermined amount from an end surface of the laminate into an interface between the first substrate and the adsorption layer so as to prepare the peeling starting portion at the interface. The knife includes a main body portion, a cutting edge portion continuous with the main body portion and tapered in a side view, and a ridge line which is a boundary between the main body portion and the cutting edge portion, and at least a part of the adsorption layer is scraped off by a ridge line portion including the ridge line.