A front bracket has a fixed part that is fixed to a bottom plate and vertical plates, and a pair of protruded parts that are protruded backward from a rear part of the fixed part so as to extend upper parts of sidewall parts. The fixed part and the protruded parts are sandwiched between opposite surfaces that are opposite to each other at front parts of the vertical plates. The protruded parts are welded to opposite surfaces of the vertical plates that are brought into contact with the protruded parts. Each welded portion between the protruded part and the opposite surface of the vertical plate is formed along an edge of the protruded part that is in contact with the opposite surface of the vertical plate.