The present disclosure provides an arm for a multi joint robot, and a multi joint robot. The arm is formed by a first half portion and a second half portion engaged with each other, an upper portion of each of the first and second half portions being provided with a protrusion, the protrusions being substantially parallel to each other, a lower portion of the first half portion being provided with a supporting member, the two half portions being engaged with each other via an engagement surface which is between the two protrusions, the shape of a lower portion of the second half portion matching the shape of an outer circumference of the supporting member.