Friction stir welding method and manufacturing method of welding structure

A friction stir welding method in which a first member to be welded formed of metal or resin and a second member to be welded formed of metal or resin and having a first step portion are arranged in a lapped manner and welded by friction stir welding using a welding tool, comprising the steps of:

• arranging the first member to be welded on a step supporting surface of the first step portion with a gap between the first member to be welded and a side surface of the first step portion;
• pushing the welding tool into the first member to be welded from a surface on the opposite side of a back surface of the first member to be welded contacting the step supporting surface while rotating the welding tool and inserting the welding tool until reaching the step supporting surface of the second member to be welded;
• and
• stirring the first member to be welded and the second member to be welded by rotating the welding tool to form a welding part,
• wherein the first member to be welded is a plate-shaped component,
• a width of the gap is 0.03 times or more to 0.2 times or less of a plate thickness of the first member to be welded,
• a welding width of the welding part formed on the surface of the first member to be welded is 0.5 times or more to twice or less of the plate thickness of the first member to be welded.