Free-viewpoint video system will provide viewers with freedom to navigate through the scene at different viewpoints. In the system, arbitrary viewpoints of videos are synthesized by the depth image-based rendering with multi-view plus depth videos. Despite the widespread of technologies for free-viewpoint video system, the field of quality assessment for the free-viewpoint video, especially the quality prediction of a synthesized image, has not yet been thoroughly investigated. This paper analyzes how distortions in color and depth images influence on the quality of a synthesized image. Then, an objective quality prediction model for a synthesized image is proposed based on the concept of intolerance of synthesis distortion. Experimental results show that the proposed model provides outstanding performance in predicting the quality of a synthesized image compared to other models.