A top sheet (210), two walls (221, 222), and two ridge portions (231, 232) are provided, tensile strengths of the top sheet (210) and the walls (221, 222) are 1470 MPa or more, each of the two ridge portions (231, 232) extend between the top sheet (210) and the walls (221, 222), in a longitudinal direction of the top sheet (210), and a cross section of a center portion in the longitudinal direction is an open cross section having a groove shape. In the cross section, the top sheet (210) includes a central groove part (260) extending in the longitudinal direction at a center of a width and a small groove part between the central groove part (260) and each of the ridge portions (231, 232), and a depth of the groove part at the center is 15 times or more and 25 times or less a sheet thickness of the top sheet (210), a depth of each of the small groove parts (240, 250) is 2 times or more and 13 times or less the sheet thickness, and an interval between each of the small groove parts (240, 250) and the ridge portion (231, 232) is 30 times or less the sheet thickness.