PURPOSE: To provide a building airtight material having sufficient followability in all directions by constituting the material from a rubber expansion joint molded into a corrugated form nearly uniform in two dimensions.

CONSTITUTION: A plurality of corrugated ridge parts 1 advancing in a certain direction and a plurality of mutually parallel corrugated ridge parts 2 crossing them at a proper angle are formed so that the main advancing directions of these ridge parts 1, 2 are parallel to the main plane of an expansion joint. The swollen parts 3 enclosed by the ridge parts 1, 2 are molded so that they are vertically swollen from the main plane of the joint alternately in longitudinal and lateral directions, and the swollen parts 3 are arranged zigzag. The flat parts on both sides of the joint are nipped by metal plates, fixed by small screws, and mounted on a required position. Thus, by two-dimensionally molding the corrugated form, the resulting material can have sufficient flexibility not only in lateral (a), longitudinal (b) and vertical (c) directions, but also in the middle direction between the b-direction and c-direction, and sufficient followability to the relative displacement between ridges.