PURPOSE: To enable a marine observation tower to be arranged at a selected area, by a method wherein a semi-floating marine observation tower which is pivotally supported to the point freely rocking on the sea-bottom base is captively held on the sea-bottom with a plurality of mooring cables.

CONSTITUTION: A mooring base 7 is fixed on the sea-bottom with piles 8, and a semi-floating marine observation tower 1 is supported to the point freely rocking via a universal joint 6 on the base 7 and is captively held on the sea-bottom 22 with a plurality of mooring lines 9. An upper column 2, a rest-house 15 and others located above the sea level 20 are connected to the ground level with a walking bridge 16 or a floating pier. Thus, a marine observation tower with good workability and without defining a construction-feasible water area can be obtained.