A joining method for joining a first plate material (11; 21; 31; 41; 51; 61; 71) to a second plate material (12; 22; 32; 42; 52; 62; 72) by screwing a self-tapping screw into the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) from one direction, the joining method comprising: putting the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) on top of each other such that a first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a) formed in at least one of the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) is opened in a contacting face (F11; F21; F31; F41; F51; F61; F71) between the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72), the first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a) being formed in a part into which the self-tapping screw is screwed in the at least one of the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72), the first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a) having an inner periphery larger than an external-thread outside diameter of the self-tapping screw; and penetrating the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) with the self-tapping screw, the first plate material (11; 21; 31; 41; 51; 61; 71) being placed on a near side in a screw-in direction of the self-tapping screw relative to the second plate material (12; 22; 32; 42; 52; 62; 72), wherein an excess part generated from the first plate material (11; 21; 31; 41; 51; 61; 71) at a time when the self-tapping screw penetrates through the first plate material (11; 21; 31; 41; 51; 61; 71) flows into the first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a); wherein the first recessed portion (61a; 71a) is formed in a cylindrical shape by press molding in the first plate material (61; 71) that is a plate material on a nearest side in the screw-in direction of the self-tapping screw, characterised in that a projecting part of the first plate material (61; 71) toward the near side of the first plate material (61; 71) in the screw-in direction is formed to have a diameter equal to or more than a diameter of a seating face of the self-tapping screw, by the press molding of the first recessed portion (61a; 71a). putting the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) on top of each other such that a first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a) formed in at least one of the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) is opened in a contacting face (F11; F21; F31; F41; F51; F61; F71) between the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72), the first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a) being formed in a part into which the self-tapping screw is screwed in the at least one of the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72), the first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a) having an inner periphery larger than an external-thread outside diameter of the self-tapping screw; and penetrating the first plate material (11; 21; 31; 41; 51; 61; 71) and the second plate material (12; 22; 32; 42; 52; 62; 72) with the self-tapping screw, the first plate material (11; 21; 31; 41; 51; 61; 71) being placed on a near side in a screw-in direction of the self-tapping screw relative to the second plate material (12; 22; 32; 42; 52; 62; 72), wherein an excess part generated from the first plate material (11; 21; 31; 41; 51; 61; 71) at a time when the self-tapping screw penetrates through the first plate material (11; 21; 31; 41; 51; 61; 71) flows into the first recessed portion (11a; 22a; 31a; 42a; 51a; 61a; 71a); wherein the first recessed portion (61a; 71a) is formed in a cylindrical shape by press molding in the first plate material (61; 71) that is a plate material on a nearest side in the screw-in direction of the self-tapping screw, characterised in that a projecting part of the first plate material (61; 71) toward the near side of the first plate material (61; 71) in the screw-in direction is formed to have a diameter equal to or more than a diameter of a seating face of the self-tapping screw, by the press molding of the first recessed portion (61a; 71a).