To make a submersible machine smoothly reach the target point in a shortest time without stopping delivery of a tether cable.

Catenary simulation of a tether cable 2 in relation to the target point is performed, the catenary curved line obtained by the catenary simulation of the tether cable 2 is taken as a simulation locus 3 of a submersible machine 1. The submersible machine 1 is made reach the target point by the automatic operation so controlling the submersible machine 1 that the actual locus approximate line approximate to the actual locus 4 near the passing point of the present time on the actual locus 4 as the actual locus of the submersible machine 1 may approach the axis of ordinates in which the axis of ordinates is overlapped on the simulation approximate straight line approximate to the simulation locus 3 near the passing point of the present time on the simulation locus 3.