The copy-move forgery is a common way to manipulate the digital images, which copied regions come from the same image. The detection process of altered regions is known as forgery detection. In this paper, we propose a new block-based approach for detecting copy-move forgery in digital images. The novelty of the proposed approach is the usage of structural similarity index as the method for similarity matching step. Additionally, Gaussian pyramid decomposition method allows us to improve run-time speed of the approach considerably. Therefore, feature extraction would not require, hence; it can help to diminish time-consuming issue. Accordingly, simplicity in calculating and analyzing is the significant specification of the proposed approach. Experiments are provided to illustrate an efficiency of suggested approach and evaluate its robustness and sensitivity against some post-processing operations.