Depression has been one of the serious social problems of the modern society, due to the prolonged economic stagnation, the increasing number of unemployment rate, and so on. Diseases that require long-term treatments, such as cancer and chronic diseases, are also the major cause of depression, and, eventually, that often leads the patients to committing suicide. Such circumstance requires some urgent means to prevent depression, especially of those who are suffering diseases. In this study, to propose one of the tools for preventing depression or to detect depression at its early stage of suffering, we analyze 200 blog articles to set-up an "evaluation index" that is capable of measuring the tendency of depression from written texts of the long-term patients. Firstly, we collect and analyze blog articles written by the patients who are suffering depression, as well as the articles written by those who are not suffering depression as a comparison. From the analysis, we propose an index for depression measurement. With this index, secondly, we make an analysis on the articles that are posted on a special SNS, which is particularly available for long-term patients. It comes out that posting articles on SNS for more than 12 months may mitigate depression (or depressive symptoms). This outcome suggests the possibility of SNS posting against depression for the long-term patients.