A work apparatus for a work on a wall surface of a structure, e.g., a building, comprises rail supports secured fixedly to an upper end portion of a structure, upper provisional horizontal rails supported provisionally on these rail supports, lower provisional horizontal rails mounted provisionally on a lower portion of a wall surface of the structure in parallel with the upper provisional horizontal rails, upper horizontally running trolleys mounted slidably on the upper provisional horizontal rails and connected to one another, a working platform, e.g., a moving scaffold, hung from the upper horizontally running trolleys in a manner to be movable in the vertical direction, a hanging frame hung from the upper horizontally running trolleys, a protecting material such as a protecting net hung from the hanging frame in a manner to enclose the working platform, a protecting material platform connected to the lower end of the protecting material, and lower horizontally running trolleys secured to the protecting material platform and mounted slidably on the lower provisional horizontal rails. In this work apparatus, a portion of the structure covered by a protecting net is minimized so that a work at an elevated location can be done by covering only a minimal portion necessary for the work.