fig1 the robot 1 is a vertical articulated robot or a known robot manipulator and at a tip part 2 of its arm a hand 3 to hold a workpiece w is attached note that the hand 3 does not necessarily have to be attached to the tip part 2 of the arm of the robot 1 the hand 3 is selected as adequate depending on the shape material mass and so on of the workpiece w the hand 3 may be either the suction model or the chuck model furthermore a force sensor 5 is provided between the tip part 2 of the arm and the hand 3 in fig1 two tables t 1 and t 2 are placed in the operation range of the robot 1 and a workpiece w is mounted on one table t 1 furthermore as illustrated in fig1 a robot controller 9 which controls the entirety of the robot 1 is connected to the robot 1 note that the force sensor 5 is also connected to the robot controller 9 the robot controller 9 is a digital computer and has a mass estimation unit 11 that estimates the mass of the workpiece w and an acceleration acquiring unit 12 that calculates the acceleration that works on.