PURPOSE: To efficiently perform a sea bed excavation and filling back with receiving no influence of maritime phenomena by a method wherein after a trench is excavated by traveling an excavating machine connected with a power line, on the sea bed by means of a signal from a land base, subaqueous cable is laid and along with this, filling back operation is performed on the above-mentioned excavation field.

CONSTITUTION: While an excavating machine 3 is traveled on the sea bottom 1 along an excavation line measured by a signal from a control-monitoring-operating board on a land base, an excavated trench 33 having a given depth and width is excavated. And in conjunction with this excavation, an electric cable 24 is laid in the trench 23, and along with this, the excavated sediment is sucked and carried to the rear side through a dredge sand pipe 35 and a dredge sand pump 36 and further, it is used to fill back the trench 33 at the rear side of the main body 10 through a sediment pipe 37. In this way, the unattended operating machine 3 continuously performs the trench excavation and filling back operation on the sea bed 1 through a remote control from the base. And after it reaches a destination, a power line 9 is disconnected from the operating machine 3 and after the construction is over, it is wound up by a power line which in the base.