PURPOSE:To laminate plastic sheets without previously carrying out embossing or the like on them by a method wherein a third plastic flat sheet is put upon a laminate, which is produced by ultrasonic-welding a first and a second plastic flat sheets together, so as to be ultrasonic-welded to the laminate by being given ultrasonic vibration with a second ultrasonic horn pressed from above. CONSTITUTION:Two square plastic flat sheets 11 and 12 are piled up and ultrasonic-welded by contact-bonding an ultrasonic horn 21 onto the plastic sheet 12. The pressing surface of the ultrasonic horn 21 has a plurality of projected parts 31 and recessed parts 33, each of which surrounds the projected part. On the top surface of the plastic flat sheet 12, protuberances 41 are produced by the plastic, which melts out of the portion coming into contact with the projected part 31 of the ultrasonic horn into the portion corresponding to the recess part 33 of the horn. A third plastic flat sheet 14 having a window at its center is placed on the laminate consisting of the two plastic flat sheets 11 and 12, which are welded together, and ultrasonic-welded together by contact- bonding an ultrasonic horn 23 having a flat pressing surface onto the plastic sheet 14.