A water-chamber working apparatus 1 according to the present invention includes a movable body that can move along a tube plate 12 of a steam generator 10, an extendable member 21 that extends and retracts in a direction in which a first coupling portion 21d approach each other and a direction in which these portions move away from each other, where the first coupling portion 21d is attached to a maintenance hatch 15 via a first joint 23a including two rotation axes intersecting with each other, and the second coupling portion 21e is attached to the movable body via a second joint 23b including two rotation axes intersecting with each other, which are different from the rotation axes of the first joint 23a, an extrusion spring 22 that applies a force in a direction in which the first coupling portion 21d and the second coupling portion 21e move away from each other to the extendable member 21, and a supporting electric motor 24 and a counterweight mechanism 25 that apply a force of rotating the extendable member 21 while using the first joint 23a as a fulcrum, to the extendable member 21 so that the second coupling portion 21e approaches the tube plate 12.