A working machine comprising a boom, an arm and a pipe mounting structure for mounting a plurality of pipes on an outer peripheral surface of at least one of the boom and the arm, wherein the boom is liftably mounted to a base machine and the arm is mounted at a tip end of the boom to be able to rotate about a horizontal axis, the pipe mounting structure comprising: a mounting portion (50; 150), to which the plurality of pipes are mounted, the mounting portion being provided to at least one of the boom and the arm, wherein the boom and the arm respectively have an upper plate (25b4), a lower plate provided at a position separated from the upper plate (25b4) in a downward direction, and a pair of side plates (25b3) arranged on both sides in a width direction of the upper plate (25b4) and the lower plate, end portions of the upper plate (25b4), the lower plate or the side plates (25b3) protruding outwardly to form at least a pair of protruding portions, wherein both side edge portions of the upper plate (25b4) and the lower plate are welded to the pair of side plates (25b3), respectively, so as to form a tubular body, and respective upper or lower end edges of the pair of side plates (25b3) protrude upward or downward beyond the upper plate (25b4) or the lower plate to form the pair of the protruding portions, or the upper end edge and the lower end edge of the side plate (25b3) respectively protrude upward and downward beyond the upper plate (25b4) and the lower plate to form the pair of the protruding portions, or wherein upper end edges and lower end edges of the pair of side plates (25a1) are welded to the upper plate (25a2) and the lower plate, respectively, so as to form a tubular body, and respective both side edges of the upper plate (25b4) or the lower plate protrude outside the side plates (25b3) in the width direction of the upper plate (25b4) to form the pair of the protruding portions, or the side edge of the upper plate (25b4) and the side edge of the lower plate respectively protrude in the width direction of the upper plate (25b4) to form the pair of the protruding portions; a bridge member, the plurality of pipes being fixed to the bridge member; characterized in that at least one bracket (46, 47, 48, 146) is fixed onto the upper surface of the bridge member by bolts and the pipes are fixed between the at least one bracket (46, 47, 48, 146) and the bridge member, the bridge member is bridged between the pair of protruding portions, and that the pipe mounting structure further comprises a connecting portion which connects the protruding portions and end portions of the bridge member to each other. a mounting portion (50; 150), to which the plurality of pipes are mounted, the mounting portion being provided to at least one of the boom and the arm, wherein the boom and the arm respectively have an upper plate (25b4), a lower plate provided at a position separated from the upper plate (25b4) in a downward direction, and a pair of side plates (25b3) arranged on both sides in a width direction of the upper plate (25b4) and the lower plate, end portions of the upper plate (25b4), the lower plate or the side plates (25b3) protruding outwardly to form at least a pair of protruding portions, wherein both side edge portions of the upper plate (25b4) and the lower plate are welded to the pair of side plates (25b3), respectively, so as to form a tubular body, and respective upper or lower end edges of the pair of side plates (25b3) protrude upward or downward beyond the upper plate (25b4) or the lower plate to form the pair of the protruding portions, or the upper end edge and the lower end edge of the side plate (25b3) respectively protrude upward and downward beyond the upper plate (25b4) and the lower plate to form the pair of the protruding portions, or wherein upper end edges and lower end edges of the pair of side plates (25a1) are welded to the upper plate (25a2) and the lower plate, respectively, so as to form a tubular body, and respective both side edges of the upper plate (25b4) or the lower plate protrude outside the side plates (25b3) in the width direction of the upper plate (25b4) to form the pair of the protruding portions, or the side edge of the upper plate (25b4) and the side edge of the lower plate respectively protrude in the width direction of the upper plate (25b4) to form the pair of the protruding portions; a bridge member, the plurality of pipes being fixed to the bridge member; characterized in that at least one bracket (46, 47, 48, 146) is fixed onto the upper surface of the bridge member by bolts and the pipes are fixed between the at least one bracket (46, 47, 48, 146) and the bridge member, the bridge member is bridged between the pair of protruding portions, and that the pipe mounting structure further comprises a connecting portion which connects the protruding portions and end portions of the bridge member to each other.