In manufacturing an insulating box used for a low-temperature show case having a slidable transparent windows on the top opening thereof, a recess is formed on the upper front ends of inner and outer boxes of an insulating box, said recess being provided with a frame covering the gaps between said inner and outer boxes and having a groove for receiving therein a multi-layer transparent panel. After the multi-layer transparent panel is installed in the groove and fixed with a fixation frame, an expandable insulating material is injected into the spaces between said inner and outer boxes to form said insulating box.