To provide a lightweight aseismatic structure which can aseismically strengthen a building in a shape with delicacy without decreasing the utility value of the building, which can be inexpensively and easily constructed in a short period of time, which can be finely adjusted depending on a load and which enables a change in strengthening after construction, and an aseismic strengthening method.

The aseismatic structure characteristically comprises: a curved crustal pipe 3 which constitutes a curved strengthening body filled with a filler and having a shape including an oval or U-shaped curved arc when it is seen from a longitudinal side surface, and which aseismically strengthens a side surface of the building 1 by the curved strengthening body; and a connecting body 5 which connects the pipe 3 to the building 1 or a vertical column 4. The aseismic strengthening method is mainly characterized as follows: the curved strengthening body, which has the shape including the oval or U-shaped curved arc when it is seen from the longitudinal side surface, is composed of the pipe 3; and aseismic strengthening is performed by connecting the curved strengthening body to the building 1 or the vertical column 4 by means of the connecting body 5.