PURPOSE: To raise the safety of a marine structure as well as to reduce its cost by a method in which a cement mortar layer into which glass fiber meshes are buried is laminated on the outer surface of an expanded synthetic resin outside shell.

CONSTITUTION: A glass fiber mesh 2 is laid on the outer surface of an outside shell 1 of a ship, a boat, or other marine structure, and a cement mortar 3 is sprayed onto the glass fiber mesh 2 to form a marine structure A. In this case, the same operations are repeated to laminate the cement mortar layer in which a given number of glass fiber meshes 2 are buried on the outside shell 1. Accordingly, the cement mortar layer 3 is energized to be restored to its original state in such a way as to contract the cracks even when cracks develop. The safety of the marine structure can thus be maintained.