A joining method for joining a first steel member and a second steel member through joining metal based on rotational friction, comprising: positioning said first steel member and said second steel member at neighboring positions with an end surface of said first steel member and an end surface of said second steel member opposing each other; manufacturing a void which straddles the end surface of said first steel member and the end surface of said second steel member, said void having a straight line perpendicular to or quasi perpendicular to a surface (12, 22) of said first steel member and said second steel member as a rotational axis, said void also having a side peripheral surface and a bottom in a rotationally symmetrical shape; inserting a joining metal into said void, said joining metal includes a joining metal main body having a rotationally symmetrical side peripheral surface whose generatrix is a monotonously changing curve and a tip portion having a conical tip end surface continuous with the joining metal main body with a size that can be easily inserted into the empty void; generating friction by rotating said joining metal around said rotational axis with a contact portion between a tip of said joining metal and said bottom of said void under a pressing force; generating liquefied molten metal by melting material structure near said contact portion by utilizing frictional heat by said friction; filling said liquefied molten metal into a gap between said side peripheral surface of said joining metal and said side peripheral surface of said void by utilizing said pressing force and rotational motion generated at the tip of said joining metal; subsequently, stopping rotational movement to have said liquefied molten metal solidified and be integrated with structure near said gap, thus joining said first steel member and said second steel member. positioning said first steel member and said second steel member at neighboring positions with an end surface of said first steel member and an end surface of said second steel member opposing each other; manufacturing a void which straddles the end surface of said first steel member and the end surface of said second steel member, said void having a straight line perpendicular to or quasi perpendicular to a surface (12, 22) of said first steel member and said second steel member as a rotational axis, said void also having a side peripheral surface and a bottom in a rotationally symmetrical shape; inserting a joining metal into said void, said joining metal includes a joining metal main body having a rotationally symmetrical side peripheral surface whose generatrix is a monotonously changing curve and a tip portion having a conical tip end surface continuous with the joining metal main body with a size that can be easily inserted into the empty void; generating friction by rotating said joining metal around said rotational axis with a contact portion between a tip of said joining metal and said bottom of said void under a pressing force; generating liquefied molten metal by melting material structure near said contact portion by utilizing frictional heat by said friction; filling said liquefied molten metal into a gap between said side peripheral surface of said joining metal and said side peripheral surface of said void by utilizing said pressing force and rotational motion generated at the tip of said joining metal; subsequently, stopping rotational movement to have said liquefied molten metal solidified and be integrated with structure near said gap, thus joining said first steel member and said second steel member.