The present authors take such an approach to build a service mobile robot that all the robot functions are categorized into common functions and specific functions. The common functions will be assigned to a mobile robot and the specific functions will be assigned to a specific mechanism. A "portable effector" is designed for a mechanism to accomplish the specific task. The portable effector will be placed on the floor near the task space in the environment. A navigation and docking functions with the effector will be provided as common functions for the mobile robot. The robot changes the portable effector automatically according to the assigned task. In this paper, the authors describe techniques of automatic docking with portable effector for the mobile robot. They include position alignment technique proper to docking by PWS type mobile robot using simple sensor and mark, and firm holding mechanism between the robot and the effector. The docking mechanism is designed so as to allow alignment error at the time of docking. The authors illustrate portable effector changing operation using two kinds of portable effectors in the real environment.