A method for joining a first wiring board (100) covered with a conductive pattern (102) on a surface of a thin resin basic plate (104) to a second wiring board (200) covered with a conductive pattern (202) on a thin resin basic plate (204) to ensure the electric connection therebetween, comprising the steps of putting the first and second wiring boards (100 and 200) together in a confront relationship so as to adjust join predetermined portions (103 and 203) on the conductive patterns, catching the join predetermined portions (103 and 203) in the status by a pair of ultrasonic welding tools (40 and 41), and applying an ultrasonic vibration to the ultrasonic welding tools (40 or 41) to weld the conductive metals located on the join predetermined portions (103 and 203).