Arc spot welding method for joining dissimilar materials and weld joint of dissimilar materials

An arc spot welding method for joining dissimilar materials, in which a first sheet made of an Al-based material or an Mg-based material and a second sheet made of ultra-high tensile steel having a tensile strength of 1180 MPa or more are joined, the arc spot welding method comprising:

• forming a hole in the first sheet;
• overlapping the first sheet and the second sheet;
• inserting a joining auxiliary member made of steel, in which a hollow portion penetrating in a sheet thickness direction of the first sheet and the second sheet is formed, into the hole provided in the first sheet;
• and
• joining the first sheet and the second sheet via the joining auxiliary member by using a welding material containing 13 mass % or more of Ni,
• wherein the joining the first sheet and the second sheet is melting the second sheet and the joining auxiliary member and melting the welding material to fill the hollow portion of the joining auxiliary member with weld metal.