To maintain and retrofit appropriately concrete structures, various strengthening materials and methods have been developed and applied. One of the effective and reliable strengthening method for concrete is an application of the prestressing system. The present study focuses on a strengthening system using an internal anchorage and a prestressing tendon. The strengthening system is acceptable even in relatively narrow workspaces, and also applicable for joints between existing and additional concrete members. In our previous investigations, a static push-out and pull-out tests were performed to examine the load-bearing capacity of the prestressing tendon embedded in the wedge anchor. The test confirmed that the prestressing tendon can be anchored firmly in an internal wedge hole filled with high-strength mortar. Long-term behavior of the strengthened member should be examined to confirm the applicability of the system. The load and deformation of a concrete member subjected to sustained force by the prestressing bar were measured for 1 year. This paper reports the long-term loading test, and discusses the time-dependent properties of the strengthened concrete member. The test result confirms that the loss of prestressing force is negligible for actual applications.