PURPOSE: To form a four-node chain constitution, and enable the efficient opening and closing of a door body by applying the constitution wherein the door body having an inner door piece pivotally fitted with a freely inside foldable outer door piece is so fitted to the front of a box part as to be capable of free opening and closing, and the turning arm of the outer door piece is pivotally supported at a turning point on a bisector between both pivotal points for door opening and closing.

CONSTITUTION: A support piece 14 extended toward an inner door piece 4 is fixed to the upper end of the outer door piece 5 of a door body 3. One end of a turning arm 6 is pivottaly connected to a pivotal point K2 at the tip of the piece 4. The other end of the piece 4 is fitted with a support axis 10 passed vertically through a top plate and horizontally turning a turning arm 6 about a turning point K3 at the center. The turning point K3 is located on the vertical bisector X2 of a line X1 between a pivotal point K21 in a door close state Y1 and a pivotal point K22 in a door open state Y2. A still link LO connecting a support point K1 to the turning point K3, a link, L1 connecting the turning point K3 to the pivotal point K2, a link L2 connecting the pivotal point K2 to the inner door piece 4 and the pivotal point K4 of the outer door piece 5 and a link L3 connecting the pivotal point K4 to the support point K1, respectively form four-node chain. By forming each section of the chain to unequal length, it is possible to open and close the door between the door close/open states Y1 and Y2 in various opening operations.