In this paper, we propose design policy for legged robots system software architecture. Quadruped robot systems is exemplified as the target legged robot, and its functional units are decomposed into some reusable subsystems with generic interfaces for legged robot. These subsystems are implemented as RT-Components, which are modular software components based on RT-Middleware. Finally, in order to show the validity of the proposed design policy, some experiments are conducted with two different quadruped robots and three different gait patterns.