Timber has been widely adopted as a global structural material. However, timber sometimes deteriorates due to various factors such as termite infestation, which may reduce its strength and durability. It is difficult to ascertain the degree of deterioration of timbers by visual inspection because deterioration by termites causes severe damage to the inner parts of the timber. Therefore, the damage is often discovered after the deterioration progresses. The restoration and refurbishment method currently implemented is removing the degraded parts and partially replacing them with new healthy wood. Although it is as desirable to reinforce the original part as it is to restore its strength, especially for historical timber structures, this approach has not yet been put into practical use. This study investigates the possibility of restoring timber deteriorated by termites with little changes in its appearance by using resin filling. Deterioration by termites occurs at the base of the timber. Therefore, the specimen is assumed to be the joint of a foundation and column. First, the deteriorated specimen was filled with epoxy resin, and then, the effectiveness of the filling was confirmed by a nondestructive test method using ultrasonic pulse velocity. Finally, shear strength test was performed for the joint. As a result, ultrasonic velocity significantly increased because of the resin filling. The strength of the specimen filled with epoxy resin was restored and was close to the strength of a healthy one. It was confirmed that this strengthening method with resin filling was useful for improving the structural performance of timber deteriorated by termites with little change in appearance.