To provide a ventilation structure and a ventilation method for a heat insulating block with a leg, laid on a roof slab etc., which can easily lower the temperature of an air layer on the downside of each heat insulating block with the leg easily in summer.

In this heat insulating structure in which the plurality of heat insulating blocks with the leg are lengthwise and crosswise laid, a rise in the temperature of the air space is suppressed because high-temperature air at the increased temperature ascends and flows out from a vent hole on ground that the vertically-penetrating vent hole is formed in the center of the at least one heat insulating block with the leg. When the vent hole-formed heat insulating blocks with the leg are arranged in such a manner as to be dispersed in a plurality of locations, the temperature is smoothly decreased by virtue of an improvement in the ventilation of the air space because outside air flows in through some of the vent holes and flows out from some of the vent holes, except them.