PURPOSE: To excavate the bottom of water with good accuracy without over- excavation and incomplete excavation by a method in which a water pressure sensor is provided to the back hoe bucket of a back hoe type dredger to inform an operator of the depth of the bucket under water.

CONSTITUTION: A slewing base 2 is mounted horizontally on a dredger 1, and the basal end of the base boom 5 of a working arm 3 having an excavator 4 on its tip is pivotally supported on the slewing base 2 in a rocking manner around the pivotal supporting point 6. Also, an arm 9 is pivotally supported at the tip of the base boom 5 in a rocking manner, and a bucket 4 is pivotally supported at the tip of the arm 9 in a rocking manner. A water depth sensor 15 is also provided at the tip of the bucket 4 in order to transmit the sensed water depth to the operation room of the dredger 1 to notice an operator of the water depth.