It is important to know the stress of concrete acting on existing structures in order to maintain structures appropriately. However, it is difficult to know the actual stress for several reasons. The major reason is lack of the design documents. Even when design documents exist, usually, actual stress conditions are different from design conditions. The causes are internal restrictions due to creep and shrinkage, differences between design assumptions and actual behaviors of structures, and so on. In this study, the mechanisms of internal restrictions are investigated through various experiments. And a new effective stress estimation method considering time-dependent behavior of concrete is proposed. The method is based on stress release technique. It can separate strain due to internal restriction from the total released strain, and the accuracy of estimated stress has been improved. From application tests, it was confirmed that the method is useful for setting maintenance level of structures.