PROBLEM TO BE SOLVED: To provide a power assist device and its adaptive model prediction control method for suppressing deviation from a target final position to small extent as much as possible by smallest possible operation force in cooperation work of a man and a robot. SOLUTION: This power assist device is provided with an articulated arm robot 10 having a fingertip part 11 for gripping an object 1 to be transported rigidly and a controller 20 for controlling the robot. The robot 10 has a sensor for detecting revolute joint angle vector q of multi-revolute joint, a fingertip part position x, and force f e that a worker working in cooperation with the robot applies on the object to be transported. The controller 20 infers coefficient of viscosity of worker's fingertip D e , its modulus of elasticity K e , and a natural length position x e of worker's fingertip based on the revolute joint angle vector q, the fingertip part position x, and worker's force f e detected by the sensor on real time. The robot 10 generates assist force to suppress positioning error to small extent as much as possible by smallest possible operation force based on the inferred values. COPYRIGHT: (C)2007,JPO&INPIT