an ultrasonic welder includes an anvil , a horn configured to sandwich a first member placed on the anvil and a second member placed on the first member with the anvil , and transmit ultrasonic vibration to the second member , so as to join the first and second members , a vibration detector configured to detect a condition of the vibration transmitted to the first member from the second member , and a controller configured to stop the ultrasonic vibration of the horn according to the detected condition of the vibration .