Reinforced carrier device (1, 1') for a battery pack of an electric or hybrid vehicle, comprising at least: - a carrier device (2, 2') comprising at least one side wall (3, 4, 5, 6, 6') and a base wall provided to receive a plurality of battery modules of the battery pack, - a reinforcement structure of said side wall (3, 4, 5, 6, 6') comprising at least an inner reinforcement piece (14, 15, 16, 17, 18a, 18b) and an outer reinforcement piece (19, 20, 21, 22, 23a, 23b) extending along at least a part of the side wall, at least partially facing each other, and being respectively secured to opposite faces of said side wall (3, 4, 5, 6, 6') then forming a reinforcement arrangement (9, 10, 11, 12, 13a, 13b), wherein the reinforcement arrangement (9, 10, 11, 12, 13a, 13b) comprises: • at least two adjacent reinforcement hollow portions (24, 25) both being traversed by the side wall (3, 4, 5, 6, 6') of the carrier device (2, 2') and each made of an inner reinforcement hollow section (26, 27) of the inner reinforcement piece (14, 15, 16, 17, 18a, 18b) and an outer reinforcement hollow (28, 29) section of the outer reinforcement piece (19, 20, 21, 22, 23a, 23b), both reinforcement hollow sections being at least partially facing each other, and • a longitudinal reinforcement fastening portion located between the adjacent reinforcement hollow portions (24, 25), secured to the side wall (3 - 6, 6'), and made of an inner reinforcement fastening section of the inner reinforcement piece (14 - 17, 18a, 18b) and an outer reinforcement fastening section of the outer reinforcement piece (19 - 22, 23a, 23b), both reinforcement fastening sections (31, 32) being at least partially facing each other. - a carrier device (2, 2') comprising at least one side wall (3, 4, 5, 6, 6') and a base wall provided to receive a plurality of battery modules of the battery pack, - a reinforcement structure of said side wall (3, 4, 5, 6, 6') comprising at least an inner reinforcement piece (14, 15, 16, 17, 18a, 18b) and an outer reinforcement piece (19, 20, 21, 22, 23a, 23b) extending along at least a part of the side wall, at least partially facing each other, and being respectively secured to opposite faces of said side wall (3, 4, 5, 6, 6') then forming a reinforcement arrangement (9, 10, 11, 12, 13a, 13b), • at least two adjacent reinforcement hollow portions (24, 25) both being traversed by the side wall (3, 4, 5, 6, 6') of the carrier device (2, 2') and each made of an inner reinforcement hollow section (26, 27) of the inner reinforcement piece (14, 15, 16, 17, 18a, 18b) and an outer reinforcement hollow (28, 29) section of the outer reinforcement piece (19, 20, 21, 22, 23a, 23b), both reinforcement hollow sections being at least partially facing each other, and • a longitudinal reinforcement fastening portion located between the adjacent reinforcement hollow portions (24, 25), secured to the side wall (3 - 6, 6'), and made of an inner reinforcement fastening section of the inner reinforcement piece (14 - 17, 18a, 18b) and an outer reinforcement fastening section of the outer reinforcement piece (19 - 22, 23a, 23b), both reinforcement fastening sections (31, 32) being at least partially facing each other.