PURPOSE: To provide the installation system of a lengthy body preventing a curved slack section formed to absorb movement on the lengthy body such as a cable or a fluid transportation pipe installed in the sea from a floating facility from losing the restoring force when the floating facility on the sea is repeatedly moved by a tidal current or waves.

CONSTITUTION: The upper section of a curved slack section 3a formed on a lengthy body 3 such as a cable or a fluid transportation pipe installed in the sea from a floating facility 1 on the sea is hooked on a mooring cable 10 fitted to the floating facility 1. Tension is invariably applied to the mooring cable 10 by a weight 4 installed in the sea, thus the curved slack section 3a is restored to the original state when the moved floating facility 1 is returned to the original position.