To provide a mold device of a cover body for an underground structure in which cooling of the corner sections in the cover body is accelerated because a burr functioning as a cooling fin is projected at the corner of the upper face of the cast cover body and no thermal strain is generated between the corner section and the straight part of the cast cover body, because the corner section of the cast cover body and the straight part are cooled at substantially the same cooling rate and hence, as the straight part is neither curved outward nor bulged out, the cutting process of the bulged part which is necessitated in conventional cases is dispensed with.

A casting cavity to form a cover body for a C, C box is formed by at least a pair of molds and the forming face for the upper face of the cover body of the casting cavity is set to coincide with the partitioning face of the pair of molds. A space for forming the burr is defined on the partitioning face and continuously connected to the corner of the casting cavity for the cover body.