A T-shaped joint structure provided with a first member and a second member joined to the first member and extending in a direction vertical to a longitudinal direction of the first member, wherein the second member is provided with a vehicle width inside part with a top side line (Li) extending in the vehicle lateral direction and vertical to the longitudinal direction of the first member, a second member side joining element as a part of the second member is connected to the outside of the vehicle width inside part in the direction vertical to the longitudinal direction of the first member, and in a side view of the first member in the longitudinal direction, a top side line (Ls) of a first member side joining element as a part of the first member, joined to the second member, is slanted toward the outside of the second member to the top side in the vertical direction of both the first and second members so that the first member tapers, and is connected to a top side line (Lf) of a top surface flange of the second member side joining element, wherein the top side line (Ls) of the first member side joining element and the top side line (Lf) of the top surface flange overlap, the first member side joining element and the second member side joining element are smoothly joined to form a joined part, and the joined part is present at least at single side in the transversal direction of the first and second members (32, 34), and wherein the absolute value of a bending angle (φ) between the line segment comprised of a top side line (Lo) of the second member side joining element and the top side line (Lf) is 0° to 25°, and a slant angle (θ) of the top side line (Lo) of the second member side joining element with respect to the top side line (Li) of the vehicle width inside part is 5° to 45°, wherein the dimension (Wi) of the vehicle width inside part in the direction vertical to the longitudinal direction of the first member is 75% to 95% with respect to the overall dimension of the second member in the direction vertical to the longitudinal direction of the first member, the overall dimension being the part at inside from the first member, wherein, in a side view of the first member in the longitudinal direction, a ratio H2/H1 of a minimum dimension H2 with respect to a maximum dimension H1 in a direction vertical to the longitudinal direction of the second member at a part of the second member other than the second member side joining element is 0.5 to 0.92, wherein when the angle (θ-φ) is made θ, (H1/H2-1)/2≦tanθ is satisfied. the second member is provided with a vehicle width inside part with a top side line (Li) extending in the vehicle lateral direction and vertical to the longitudinal direction of the first member, a second member side joining element as a part of the second member is connected to the outside of the vehicle width inside part in the direction vertical to the longitudinal direction of the first member, and in a side view of the first member in the longitudinal direction, a top side line (Ls) of a first member side joining element as a part of the first member, joined to the second member, is slanted toward the outside of the second member to the top side in the vertical direction of both the first and second members so that the first member tapers, and is connected to a top side line (Lf) of a top surface flange of the second member side joining element, wherein the top side line (Ls) of the first member side joining element and the top side line (Lf) of the top surface flange overlap, the first member side joining element and the second member side joining element are smoothly joined to form a joined part, and the joined part is present at least at single side in the transversal direction of the first and second members (32, 34), and wherein the absolute value of a bending angle (φ) between the line segment comprised of a top side line (Lo) of the second member side joining element and the top side line (Lf) is 0° to 25°, and a slant angle (θ) of the top side line (Lo) of the second member side joining element with respect to the top side line (Li) of the vehicle width inside part is 5° to 45°, wherein the dimension (Wi) of the vehicle width inside part in the direction vertical to the longitudinal direction of the first member is 75% to 95% with respect to the overall dimension of the second member in the direction vertical to the longitudinal direction of the first member, the overall dimension being the part at inside from the first member, wherein, in a side view of the first member in the longitudinal direction, a ratio H2/H1 of a minimum dimension H2 with respect to a maximum dimension H1 in a direction vertical to the longitudinal direction of the second member at a part of the second member other than the second member side joining element is 0.5 to 0.92, wherein when the angle (θ-φ) is made θ, (H1/H2-1)/2≦tanθ is satisfied.