There are a very large number of systems that have been developed on embedded operating system (OS) kernels. Those kernels typically do not provide protection domains because of the limitations of their target platforms. The requirements for embedded OS kernels are, however, changing. The current and future embedded systems software is becoming larger and more complex. In order to accommodate such large and complex software systems, embedded OS kernels are required to provide protection domains. This paper presents an approach, in which a microkernel is employed 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 TLA microkernel and a μITRON embedded kernel, and shows the feasibility of our approach through evaluations.