Left and right front flanges are provided respectively in left and right front legs in a center frame. Left and right rear flanges are provided respectively in left and right rear legs in the center frame. Left and right front leg mounting members, and left and right rear leg mounting members, which protrude toward the center frame are provided respectively in left and right side frames. The respective front flanges are placed on the respective front leg mounting members, which are respectively fastened by bolts inserted therebetween in the upper-lower direction. The respective rear flanges are placed on the respective rear leg mounting members, which are respectively fastened by bolts inserted therebetween in the upper-lower direction.