Embedded operating system (OS) kernels provide basic execution primitives that can be used commonly by many appliances. The recent emergence of digital appliances requires more advanced features, such as networking and GUI. Those features dramatically complicate the appliances' software systems and increase their code sizes. In order to develop such large and complex software systems, we propose a system architecture that co-locates multiple embedded operating systems on a microkernel. The proposed architecture employs a microkernel to provide protected execution environments for the existing embedded kernels. In each protected execution environment, a kernel and its applications run just as they run directly on hardware since they share the same protection domain. Our approach can achieve the maximum reusability of the existing software resources including embedded OS kernels and their applications; thus, it protects the existing software resources. We have developed the system that consists of TL4 microkernel and a μITRON embedded kernel, and shows the feasibility of our approach through evaluations.