Structural Health Monitoring (SHM), the technology to evaluate the health of the structure (such as earthquake resistance), is getting attention recently. The techniques for evaluating earthquake-proof performance in the building are key elements. However, existing techniques are not so practical as they require professional knowledge on structural engineering. The chaos analysis method, recently proposed by Prof. Okada1) is an attractive method to meet the need. The method evaluates earthquake-proof performance, using chaos analysis. It is clear that there is correlation between chaos theory and physical parameters. This paper proposes a modified version of chaos analysis to make the method stable and reliable. Findings in the course of this study are as follows. (1) The chaos degree rises when attenuation grows. (2) The chaos degree rises by widening the band of the frequency. (3) The ground motion with large power decides the chaos degree. (4) The chaos degree is not unique for the same structure depending on the inputs. These are due to the results of simulation and full-scale experiments. Using these findings, the computational method of the chaos degree without depending on the characteristics of inputs is established. It is a method using the system identification to estimate the parameter model of building from the result of the response due to any inputs. The influence of the input characteristic can be excluded by this method.