An method for ultrasonic-sealing is a method in which a plurality of seal portions (S) are formed in a continuous web (30) at an interval in a direction of transporting the continuous web (30), the continuous web (30) being for an absorbent article, the forming being performed by repeatedly executing ultrasonic-sealing processing on a continuous web (30) that is being transported. The ultrasonic-sealing processing includes: a clamping step of clamping the continuous web (30) with an ultrasonic horn (8) and an anvil (14)； a seal-portion forming step of forming the seal portions (S) in the continuous web (30) by vibration of the ultrasonic horn (8) due to receiving an ultrasonic wave generated by an ultrasonic wave generator (9)； and an unclamping step of unclamping the continuous web (30). In the seal-portion forming step, the ultrasonic wave generator (9) generates the ultrasonic wave such that a period of time from when the generation of the ultrasonic wave starts until when the generation of the ultrasonic wave ends is longer than a repetition cycle of the ultrasonic-sealing processing.
