To dismantle a radiation polluted structure (a material S to be cut) discharged from a nuclear power plant or the like by efficiently and safely cutting it in water.

The underwater cutting device 1 is provided with a device trestle 2, which can be installed in water, and a base body 3 provided movably to the device trestle 2. A conveying part 4, which positions the material S to be cut to a cutting position by gripping it freely attachably/detachably, is provided on the device trestle 2. A driving wheel 10 and a sliding driven wheel 12, which support an endless strip saw blade 15 freely rotatably in an inclined state, are provided on the base body 3.