An arm for a construction machine formed as a box-shaped structural body having a square cross sectional surface having left and right side plates (12, 13), an upper plate joined to the upper end sides of said left and right side plates (12, 13) by welding, a lower plate joined to the lower end sides of said left and right side plates (12, 13) by welding, and a rear plate joined to the rear end sides of said left and right side plates (12, 13) and the rear end side of said upper plate by welding, comprising: a boom connecting boss located on the rear parts on the lower sides of said left and right side plates (12, 13) and joined to the rear ends of said left and right side plates (12, 13) and said lower plate and the front end of said rear plate by welding; a bucket connecting boss joined to the front ends of said left and right side plates (12, 13), said upper plate, and said lower plate by welding; and a pair of left and right bucket cylinder brackets joined to an outer surface of said upper plate by welding, wherein, said left and right side plates (12, 13) are formed by joining two members, that is, a rear thick side plate (12A, 13A) located on the rear side and made of a plate material having a large plate thickness and a front thin side plate (12B, 13B) located on the front side of said rear thick side plate (12A, 13A) and made of a plate material having a small plate thickness; characterized in that; said upper plate is formed by joining two members, that is, a rear thick upper plate located on the rear side and made of a plate material having a large plate thickness and a front thin upper plate located on the front side of said rear thick upper plate and made of a plate material having a small plate thickness; said lower plate is formed by joining two members, that is, a rear thick lower plate located on the rear side and made of a plate material having a large plate thickness and a front thin lower plate located on the front side of said rear thick lower plate and made of a plate material having a small plate thickness; said pair of said bucket cylinder brackets is configured to be joined to an outer surface of said rear thick upper plate; said boom connecting boss is configured to be joined to the rear end of said rear thick lower plate; and a joint portion between said rear thick side plate (12A, 13A) and said front thin side plate (12B, 13B) is configured such that an upper end (12D, 13D) thereof is joined to said rear thick upper plate and a lower end (12E, 13E) is joined to said rear thick lower plate; wherein an auxiliary welding member having a flat plate shape is provided on an outer surface (14A4) of the rear part side of said upper plate so as to surround a welded portion between said pair of left and right bucket cylinder brackets and said upper plate; and a welding bead is formed by applying fillet welding around said auxiliary welding member. a boom connecting boss located on the rear parts on the lower sides of said left and right side plates (12, 13) and joined to the rear ends of said left and right side plates (12, 13) and said lower plate and the front end of said rear plate by welding; a bucket connecting boss joined to the front ends of said left and right side plates (12, 13), said upper plate, and said lower plate by welding; and a pair of left and right bucket cylinder brackets joined to an outer surface of said upper plate by welding, wherein, said left and right side plates (12, 13) are formed by joining two members, that is, a rear thick side plate (12A, 13A) located on the rear side and made of a plate material having a large plate thickness and a front thin side plate (12B, 13B) located on the front side of said rear thick side plate (12A, 13A) and made of a plate material having a small plate thickness; characterized in that; said upper plate is formed by joining two members, that is, a rear thick upper plate located on the rear side and made of a plate material having a large plate thickness and a front thin upper plate located on the front side of said rear thick upper plate and made of a plate material having a small plate thickness; said lower plate is formed by joining two members, that is, a rear thick lower plate located on the rear side and made of a plate material having a large plate thickness and a front thin lower plate located on the front side of said rear thick lower plate and made of a plate material having a small plate thickness; said pair of said bucket cylinder brackets is configured to be joined to an outer surface of said rear thick upper plate; said boom connecting boss is configured to be joined to the rear end of said rear thick lower plate; and a joint portion between said rear thick side plate (12A, 13A) and said front thin side plate (12B, 13B) is configured such that an upper end (12D, 13D) thereof is joined to said rear thick upper plate and a lower end (12E, 13E) is joined to said rear thick lower plate; wherein an auxiliary welding member having a flat plate shape is provided on an outer surface (14A4) of the rear part side of said upper plate so as to surround a welded portion between said pair of left and right bucket cylinder brackets and said upper plate; and a welding bead is formed by applying fillet welding around said auxiliary welding member.