In an ultrasonic joining method for joining a flange portion of a first member to a portion of a second member, a first horn and a second horn are arranged on the flange portion on a side opposite to the second member, and the first horn is vibrated in a condition that the second horn is biased against the first horn and the first horn and the second horn are pressed against the flange portion such that the flange portion is vibrated while being pressed against the portion of the second member. Because the first horn is vibrated in a condition that the first horn and the second horn are pressed against each other at press-contact portions thereof, vibration of the first horn is transferred to the second horn through the press-contact portions.