A method for ultrasonic-sealing in which a seal portion is formed in a continuous web for an absorbent article, the forming being performed by vibration of the ultrasonic horn due to receiving an ultrasonic wave generated by an ultrasonic wave generator while the continuous web is being transported and clamped between an ultrasonic horn and an anvil, the method for ultrasonic-sealing comprising: a seal-portion forming step of forming a plurality of the seal portions at an interval in a direction of transporting the continuous web by repeatedly executing processing for forming the seal portion; and characterized by a frequency adjusting step of adjusting a frequency of the ultrasonic wave generated by the ultrasonic wave generator for the ultrasonic horn 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 ends the generation of the ultrasonic wave for the ultrasonic horn in one instance of processing for forming the seal portion and when the ultrasonic wave generator starts to generate the ultrasonic wave for the ultrasonic horn in a subsequent instance of processing for forming the seal portion by the ultrasonic horn, the ultrasonic wave generator starting to generate the ultrasonic wave at the frequency stored in the storage unit when the processing for forming the seal portion is to be executed. the forming being performed by vibration of the ultrasonic horn due to receiving an ultrasonic wave generated by an ultrasonic wave generator while the continuous web is being transported and clamped between an ultrasonic horn and an anvil, the method for ultrasonic-sealing comprising: a seal-portion forming step of forming a plurality of the seal portions at an interval in a direction of transporting the continuous web by repeatedly executing processing for forming the seal portion; and characterized by a frequency adjusting step of adjusting a frequency of the ultrasonic wave generated by the ultrasonic wave generator for the ultrasonic horn 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 ends the generation of the ultrasonic wave for the ultrasonic horn in one instance of processing for forming the seal portion and when the ultrasonic wave generator starts to generate the ultrasonic wave for the ultrasonic horn in a subsequent instance of processing for forming the seal portion by the ultrasonic horn, a seal-portion forming step of forming a plurality of the seal portions at an interval in a direction of transporting the continuous web by repeatedly executing processing for forming the seal portion; and characterized by a frequency adjusting step of adjusting a frequency of the ultrasonic wave generated by the ultrasonic wave generator for the ultrasonic horn 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 ends the generation of the ultrasonic wave for the ultrasonic horn in one instance of processing for forming the seal portion and when the ultrasonic wave generator starts to generate the ultrasonic wave for the ultrasonic horn in a subsequent instance of processing for forming the seal portion by the ultrasonic horn, when the ultrasonic wave generator ends the generation of the ultrasonic wave for the ultrasonic horn in one instance of processing for forming the seal portion and when the ultrasonic wave generator starts to generate the ultrasonic wave for the ultrasonic horn in a subsequent instance of processing for forming the seal portion by the ultrasonic horn, the ultrasonic wave generator starting to generate the ultrasonic wave at the frequency stored in the storage unit when the processing for forming the seal portion is to be executed.