Arc spot welding method and welding apparatus for working the same

An arc spot welding apparatus comprising;

• a backing plate provided with a non-contact part arranged separated from a back surface of said lower side steel sheet;
• and a contact part contacting the back surface of the lower side steel sheet maintaining a clearance of 0.2 t to 3 t between the back surface of said lower side steel sheet and said non-contact part, wherein
• the arc spot welding apparatus is configured to perform a method of
• arranging a plurality of steel sheets containing carbon in amounts of 0.2 mass % or more overlaid and performing lap welding while arranging the backing plate in contact with the lower side steel sheet at an opposite side to a welding torch,
• arranging the backing plate so that said contact part contacts the lower side steel sheet at a position exceeding 3 t from a circle equivalent center of a back surface weld bead and
• alternately and continuously repeating arc generation and short-circuit conduction so that a short-circuiting time where a welding voltage becomes 10V or less becomes over 30% to less than 60% per conduction cycle,
• where “t” is defined as the sheet thickness of the steel sheet with the thinnest sheet thickness among said plurality of steel sheets.