A joining system includes a jig (20). A jig (20) includes a jig body (21) including a plurality of holding holes (22) each configured to hold a corresponding one of a plurality of workpieces (W), and a lid (25) configured to, when disposed on the jig body (21), cover the plurality of holding holes (22). A heater (30) heats the jig (20). An ultrasonic vibration applying device (80) applies ultrasonic vibration to the jig (20) during the joining, thereby applying the ultrasonic vibration to the workpieces (W) via the jig (20).