A method including: a step of forming depressions and protrusions on a first continuous sheet between a first roll and a second roll； a placement step of placing a plurality of granular particles in depressed portions of the depressions and protrusions of the first continuous sheet； a step of laying a second continuous sheet on the first continuous sheet, thereby forming a sandwich structure； and a step of bonding the two sheets with each other along protruding portions of the depressions and protrusions of the first continuous sheet, wherein a circumferential velocity of the second roll is greater than a circumferential velocity of the first roll.
