An arm for a construction machine, including an arm top formed by casting, being easily produced with high dimensional accuracy. The arm includes an arm body and an arm top. The arm top is formed of left and right arm top pieces individually formed by casting and joined together. The left and right arm top pieces include first left and right bosses constituting a first connection section, second left and right bosses constituting a second connection portion, and left and right projecting wall portions constituting a connection wall connecting the first and second connection portions to each other, respectively. The left and right arm top pieces are joined together by welding at respective opposing surfaces of the first left and right bosses, those of the second left and right bosses and those of the left and right projecting wall portions, thereby forming the arm top.