For effective photon conversion in solar-thermophotovoltaic (TPV) systems, an enclosed-space confined emitter system is proposed, and its power generation potential is demonstrated. A cuboid monolithic absorber/emitter is applied for higher extraction efficiency without dead areas. An analysis using an enclosed space shows a 4.1% higher absolute system efficiency than that using a planar absorber/emitter system. In the experiment, system efficiency reaches 7.0%, which is obtained after multiplying the power measured from one cell by five. A system efficiency more than 20% is achievable by further improvement with a back surface reflecting TPV cells and a perfectly enclosed space.