An arm device includes a joint portion configured to be rotated to thereby change a position and/or a posture of a surgical tool, and an actuator configured to generate a driving force that rotates the joint portion, and to allow a rotation of the joint portion in accordance with an externally applied force. Moreover, the arm device is configured to estimate an external force applied to the surgical tool, to obtain a value of a driving force of the actuator based on a weight of the surgical tool and the external force, and to control the actuator based on a value of the driving force.
