Provided are a filling component useful for the production of a thin, low-profile three-dimensional integrated semiconductor device via a COW process, and a curable composition for the formation of the filling component. The filling component according to the present invention for three-dimensional mounting of semiconductor elements is used to fill gaps between laterally adjacent semiconductor elements in the production of a three-dimensional integrated semiconductor device by stacking and integrating semiconductor elements. The filling component is a component that is polished and/or ground flat from the front side of semiconductor elements while gaps between the semiconductor elements are filled with the filling component.