Provided is a front side frame 2, comprising: a main frame 201; and reinforcing members 30, 40, 50 disposed inside the main frame 2, wherein the reinforcing member 30 includes partition wall portions 31b, 32d, 32e dividing a main closed cross-section C_m of the main frame 201 into a plurality of sub closed-cross sections c, the first to third partition wall portions 31b, 32d, 32e being respectively formed with compression-side ridges 31s, 32s, 32u vertically facing each other and able to come into contact with each other in a deformation of the front side frame 2, and tension-side ridges 31t, 32t, 32v arranged in the same manner.