To solve a problem wherein there is a possibility of an age-based decrease in airtight performance at a part where a structure passes through a heat insulating material.

This airtight structure of a building comprises a first airtight member interposed at least at a part of an abutting part between a side face part of the heat insulating material and the structure, and a second airtight member stuck to the joint of the abutting part. Even when the airtight performance of the second airtight member decreases due to the secular lowering or the like of adhesive strength, the first airtight member interposed at least at a part of the abutting part closes up a clearance of the abutting part. The secular decrease in the airtight performance as the whole building can thereby be suppressed.