An method for ultrasonic-sealing is a method in which a seal portion (S) is formed in a continuous web (30) for an absorbent article,the forming being performed while the continuous web (30) is clamped between an ultrasonic horn (8) and an anvil (14). The method for ultrasonic-sealing includes: a frequency adjusting step of adjusting a frequency of the ultrasonic wave generated by the ultrasonic wave generator (9) for the ultrasonic horn (8) and storing the adjusted frequency in a storage unit, the adjusting and the storing being performed at a time between when the ultrasonic wave generator (9) ends the generation of the ultrasonic wave for the ultrasonic horn (8) in one instance of processing for forming the seal portion (S) and when the ultrasonic wave generator (9) starts to generate the ultrasonic wave for the ultrasonic horn (8) in a subsequent instance of processing for forming the seal portion (S) by the ultrasonic horn (8). The ultrasonic wave generator (9) starts to generate the ultrasonic wave at the frequency stored in the storage unit when the processing for forming the seal portion (S) is to be executed.
