Organs which constitute body of organisms have a unique and functional shape. Revealing the mechanism of organ formation is expected for finding the cause of the disease, and application to regenerative medicine. A shape of organs is often characterized by sheet tissue called epithelium, and tissue shape is formed by the cooperation of the dynamic cellular activities such as apical contraction and cell proliferation. As one of the tissue deformation process, invagination is well known. Invagination is a three dimensional deformation process, and observed in many important organ formation processes, for instance, neural tube, archenteron, and optic cup. In the invagination process, patterned apical contraction is observed in vivo. However, details of the mechanical aspect of invagination have not been revealed yet. Reversible network reconnection (RNR) model is proposed as a useful model for analyzing tissue deformation from a point of view of mechanics. In this study, we investigate the role of cellular activities in invagination by using a multi-cellular dynamic simulation based on RNR model. Simulation results suggest that apical contraction determine the location and direction of invagination, and cell proliferation promotes invagination process.