Mobile code is an emerging paradigm of distributed computing. It roams over a network, is linked with an application, and runs as a part of an application. In the case of web browsers, it is commonplace to download a mobile code, called a plug-in, from a truly open network such as Internet. Owning to the anonymity of an open network, the mobile code may be malicious; thus, it is indispensable to protect local computing resources from attacks by the malicious code. We have developed a kernel that supports fine-grained protection domains that preclude mobile code from making unauthorized accesses to the local resources. The developed scheme provides a novel mechanism, called a {\em multi-protection\/} page table, of virtual memory for creating fine-grained protection domains. The multi-protection page table enables efficient cross-domain calls, whereas it provides protection. Experimental results show that the developed scheme incurs only a 5.9% execution overhead even if cross-domain calls occur 30,000 times per second.