A working machine comprising a lower propelling body, an upper slewing body provided pivotably around a pivot axis set in advance on the lower propelling body, and an attachment supported swingably by a front part of the upper slewing body around two axes (VJ, HJ), which are a vertical axis parallel to the pivot axis and a horizontal axis orthogonal to the pivot axis, wherein the upper slewing body has a bottom plate pivotably supported by the lower propelling body, a pair of vertical plates stood on the bottom plate so as to face each other in a right and left direction and to be extended to a front and rear direction, a swing bracket swingably supporting the attachment around the horizontal axis, and a front bracket swingably supporting the swing bracket around the vertical axis, wherein the front bracket includes: a fixed part fixed to the bottom plate and the vertical plates, the fixed part having an upper wall part and a lower wall part opposite to each other in an up and down direction, a pair of sidewall parts opposite to each other in the right and left direction and also joining the upper wall part and the lower wall part, and a rear wall part joining a rear part of the upper wall part, a rear part of the lower wall part, characterized in that the rear wall part joins rear parts of the pair of sidewall parts, and in that the front bracket further includes: a pair of protruded parts protruded backward from a rear part of the fixed part by exceeding the rear wall part and also integrally joined respectively to the upper parts of the sidewall parts, wherein the fixed part and both of the protruded parts are sandwiched between opposite surfaces that are opposite to each other at front parts of both of the vertical plates, and wherein the protruded parts are welded to the opposite surfaces of the vertical plates that are brought into contact with the protruded parts, and each welded portion between each of the protruded parts and a corresponding one of the opposite surfaces of the vertical plates is formed along an edge of the protruded part that is in contact with the corresponding one of the opposite surfaces of the vertical plates. a fixed part fixed to the bottom plate and the vertical plates, the fixed part having an upper wall part and a lower wall part opposite to each other in an up and down direction, a pair of sidewall parts opposite to each other in the right and left direction and also joining the upper wall part and the lower wall part, and a rear wall part joining a rear part of the upper wall part, a rear part of the lower wall part, characterized in that the rear wall part joins rear parts of the pair of sidewall parts, and in that the front bracket further includes: a pair of protruded parts protruded backward from a rear part of the fixed part by exceeding the rear wall part and also integrally joined respectively to the upper parts of the sidewall parts, wherein the fixed part and both of the protruded parts are sandwiched between opposite surfaces that are opposite to each other at front parts of both of the vertical plates, and wherein the protruded parts are welded to the opposite surfaces of the vertical plates that are brought into contact with the protruded parts, and each welded portion between each of the protruded parts and a corresponding one of the opposite surfaces of the vertical plates is formed along an edge of the protruded part that is in contact with the corresponding one of the opposite surfaces of the vertical plates. a pair of protruded parts protruded backward from a rear part of the fixed part by exceeding the rear wall part and also integrally joined respectively to the upper parts of the sidewall parts, wherein the fixed part and both of the protruded parts are sandwiched between opposite surfaces that are opposite to each other at front parts of both of the vertical plates, and wherein the protruded parts are welded to the opposite surfaces of the vertical plates that are brought into contact with the protruded parts, and each welded portion between each of the protruded parts and a corresponding one of the opposite surfaces of the vertical plates is formed along an edge of the protruded part that is in contact with the corresponding one of the opposite surfaces of the vertical plates.