In long-time extennsion works of a breakwater and a wharf facing the open sea, there have been many cases where some ends and corners of the base of structure are damage befor to next work starts.This was also the case with the extension work of the East Breakwater of Hitachi-Naka Port that faces the Pacific Ocean in a severe construction. Under such circumstances, a study meeting was estalished in 1994 by the Kashima Harbor Construction Office, No.2 Harbor Construction Bureau of the Construction Ministry. The meeting is composed of learned men of experience and businessmen concerned with the port and harbor works to study and discuss some stuructures that may replace the concrete aporon block. As a result, “protective equipment of steel frame with chain net (hereinafter called a net for foundation)”, which is lighter and larger in convering area than the concrete apron block, etc, was devised and used for the actual construction.This paper descrdes the construction status, results of followup investion and those of hydraulic model experiments of the net for a foundation at East Breakwater of Hitachi-Naka Port.