PURPOSE: To build an upper structure while reducing field work, by securing cabin units of pre-fab structure completed of finish in cubic then securing radio unit and roof unit to the uppermost layer.

CONSTITUTION: When building the hull upper structure 10, each unit is pre- fablicated including interior. Cabin unit 1, public unit 2, machinery unit 3, gallery unit 4, sanitation unit 5, sailing machinery unit 6 and path unit 9 are secured cubically in predetermined positional relation. Then, radio unit 7 and roof unit 8 are secured to the uppermost layer. Consequently, field work is reduced while schedule is shortened resulting in reduction of cost.