Method for processing plate substrate

A method for processing a plate substrate, the method comprising:

• a step A of bonding a plate substrate (1) and an optically transparent plate substrate (2) through a temporary fixing composition;
• a step B of curing the temporary fixing composition to form an adhesive layer and bond the substrate (1) and the substrate (2) to each other;
• a step C of processing the substrate (1) or the substrate (2);
• and
• a step D of irradiating the adhesive layer with laser light from the substrate (2) side to decompose the adhesive layer and generate outgas, and peeling the substrate (1) and the substrate (2) from each other by the outgas, wherein
• in the step D, a part of the substrate (2) is linearly masked to form a linear region where the adhesive layer is not to be irradiated with the laser light, wherein the linear region has a shape surrounding a part or an entirety of the adhesive layer, and an area of the adhesive layer surrounded by the linear region is 90.2% or more of a total area of the adhesive layer, and
• in the step D, a ratio of a mask length to an outgas retention length is 0.15% or more, each of the lengths measured along a line connecting a peeling start point and a peeling end point.