The Robot Operating System (ROS) has attracted attention as a design platform for robot software development. One of the problems of ROS is that it is necessary to employ high-performance and power-hunger devices since ROS requires a Linux environment for operation. This paper proposes a novel solution called mROS, which is a lightweight runtime environment of ROS nodes, to execute robot software components on mid-range embedded devices. mROS consists of a real-time operating system (RTOS) and a TCP/IP protocol stack to provide a tiny ROS communication library. It provides connectivity from the edge node to the host and other nodes through the native ROS protocol. Additionally, we design mROS APIs that are compatible with ROS 1. Therefore, native ROS nodes can be ported from Linux-based systems to RTOS-based systems as mROS nodes. Experimental results confirmed that mROS meets the performance requirement for practical applications. Moreover, we showed the size of the library constituting mROS is small for target embedded devices. We further conducted a case study to validate the portability of mROS from ROS nodes. Our work is expected to contribute to the power saving and real-time performance enhancement of mobile robot systems.