A stiffened plate obtained by welding one or a plurality of stiffening materials (20) onto the surface of a steel plate (10), the stiffened plate in which a single-bevel or single-J groove (24) is formed in an end (22) of the stiffening material, which contacts the steel plate; and a root portion (26) of the single-bevel or single-J groove is subjected to laser beam welding, and a splay portion (28) to arc welding.