To provide a joint cap that can eliminate ringing with a simple structure and improve the degree of freedom in network design for an in-vehicle LAN.SOLUTION: A joint cap 20 attached to a joint portion 33 (33a or 33b) in which one ends of a plurality of signal lines 30 are bundled and electrically connected to each other includes an insulating cap body 22 provided with a hole 25 into which the joint portion 33 is inserted, and a conductive portion 23 provided on the cap body 22 at a distance from the hole 25 and grounded, and the conductive portion 23 is a plate-shaped member or the like whose thickness and area are adjusted so as to have a capacitor capacity that functions as a low-pass filter that removes a high-frequency ringing component from the signal flowing through the signal line 30 between the conductive portion 23 and the joint portion 33.SELECTED DRAWING: Figure 5