An ultrasonic bonding device includes a stage and an ultrasonic horn. A first flat member and a second flat member to be bonded are placed on the stage. The ultrasonic horn includes a press part to be pressed on a laminated portion of the first flat member and the second flat member. The stage includes a lower-side surface, a higher-side surface, and a step wall surface. The first flat member is to be placed on the lower-side surface. The higher-side surface is positioned higher than the lower-side surface by a predetermined step height. The second flat member is to be placed on the higher-side surface. The step wall surface is positioned in a boundary between the lower-side surface and the higher-side surface.