In this paper, the motions of a large vessel which is moored by the chains among the irregular sea of finite depth are studied. It is the main aim to investigate the phenomenon of slow-drift oscillation of the moorilvsystem. Theoretical and experimental results show the followings : (1) If the damping coefficient of the swaying motion near the natural frequency of the system could be corrected from freely damped oscillation test, the slow-drift oscillation may be estimated by using the already existing method to calculate the spectrum of slowly varying drift force. (2) The tensions of mooring lines are affected definitly by the slow-drift oscillation. Even in such a case, it is permissible in the design sense of the mooring syslem to calculate the maximum tension of the mooring lines from the maximum displacement of swaying using what is called “catenary theory”. (3) Finally, a design method of the mooring system is shown by way of example.