Refinement in formal specifications has received significant attention as a method to gradually construct a rigorous model. Although refactoring methods for formal specifications have been proposed, there are no methods for refactoring of refinement structures in formal specifications. In this paper, we describe a method to restructure refinements in specifications of Event-B, a formal specification method with supports for refinement. The core of our method is decomposition of refinements. Namely, when an abstract Event-B machine A, a concrete machine C refining A, and a slicing strategy are provided, our method constructs a consistent intermediate machine B, which refines A and is refined by C. We show effectiveness of our methods through two case studies on representative usages of our method: decomposition of large-scale refinements and extraction of reusable parts of specifications.