To solve the risk of damages when performing work near an existing structure underwater.

The device 1 for underwater work includes: a main body 1b of the device 1 for the underwater work; a second arm freely swingably supported and extended from the main body 1b; an extensible first arm 1d freely swingably supported at the distal end part of the second arm; a freely detachable attachment attached to the distal end part of the first arm 1d; measurement means provided on the first arm 1d for measuring the position, direction and inclination of the first arm 1d; a stroke gauge for measuring the length of an arm part and an ultrasonic distance meter for measuring a distance to an existing structure, which are provided on the first arm 1d; and an arithmetic display device provided in an operation chamber 1a of the main body 1b such that the measuring means, the stroke gauge and the ultrasonic distance meter are electrically connected respectively and respective measured data are arithmetically processed appropriately and displayed at a display part. The underwater work near the existing structure 2 is executed.