Joining-member welding unit

A joining-member welding unit comprising:

• a first member made of metal;
• a second member overlapped with one surface of the first member, the second member having a through-hole defined by an inner wall surface of the second member, the inner wall surface of the second member and the one surface of the first member defining a recessed portion;
• a joining member made of the same kind of metal as the metal of the first member, the joining member including a large-diameter plate portion and a protruding portion, the large-diameter plate portion having an outer diameter larger than a diameter of the through-hole, the large-diameter plate portion having a thickness smaller than the diameter of the through-hole, the protruding portion protruding from the large-diameter plate portion, and the protruding portion being configured to be inserted into the through-hole;
• and
• a welder including a holding portion configured to hold the joining member, and the welder configured to apply an electric current to the joining member to melt the protruding portion of the joining member such that the first member and the second member are joined together.