[Object] To provide a method for bonding composite materials with which it is possible to bond composite materials, each comprising a melting material and a heat-resistant material that greatly differ in melting point to each other, with the heat-resistant materials facing each other
[Solution] A method is used for bonding composite materials (ceramic separators 40) that are each provided with a melting material (polypropylene layer 41) and a heat-resistant material (ceramic layer 42) comprising a heat-resistant member (ceramic particles 42M) and a binder member (binder 42N). A pair of ceramic separators with the ceramic layers facing each other are bonded. An ultrasonic wave is applied to the ceramic separators while applying a pressure thereto by the processing member (ultrasonic horn 141), and heat T is applied to the binder with a heating member (heater 147). Here, the heater 147 heats the binder to a temperature that is greater than or equal to the glass transition temperature of the binder and less than the melting point of the polypropylene layer.