This paper describes calculation method of failure probability for safety factors of existing harbor and coastal facilities from viewpoint of their maintenance that has become of great importance in recent years. Design method by using safety factor had been used until 2007, when “Technical Standards for Harbor Facilities” was revised by which reliability-based design method was employed. Large amount of harbor and coastal facilities designed by using safety factor method, therefore, exist in Japan. Finding out probability density function of the ratio resistant force to acting force, which is equal to safety factor and both of the forces are random valuables, failure provability of their structures can be calculated in discipline of mathematical statistics.
