To enable loading and unloading of a grain hopper without attachment/detachment of mechanical walls of increasing volume.

The lateral width size of front and rear mechanical walls 3a, 3b is made to respond to the inside size between left and right mechanical walls 3b, 3b so that the longitudinal width size may be so set as the end parts can be overlapped with the external surfaces of the left and right end surfaces of the mechanical walls 3a, 3a. Respective lower end edge parts of the mechanical walls 3a, 3a are joined toward the upper surface of a machine frame through a connection shaft with free rising/sinking and turning motion and a part position raised by at least the thickness amount of the mechanical walls 3a, 3a from the lower edges of the mechanical walls 3b, 3b is joined toward the upper surface of the machine frame through connection shafts S2, S2 arranged in positions raised in relation to the upper edge part position by at least the thickness amount of the mechanical walls 3a, 3a from the above-mentioned connection shaft with free rising/sinking and turning motion and a releasable lock mechanism 9 is mounted between both left and right end parts of the mechanical walls 3a, 3a and both front and rear end parts of the mechanical walls 3b, 3b.