Humans perform various motion such as jumping or throwing without high vibration, via variable viscoelasticity characteristics. On the other hand, highly rigid actuators such as geared motors or hydraulic actuators are widely used in industrial robots. It is difficult that they have variable viscoelasticity for them. In addition, to obtain high-speed motion, it is necessary to increase the actuator output as the robot weight increases. We have developed a one-degree-of-freedom manipulator with a variable rheological joint using a straight-fiber-type artificial muscle and a magnetorheological (MR) brake. With the generation of instantaneous force, the dead and rise times decreased compared to the conventional method. After the generation of an arbitrary instantaneous force, we controlled the robot's arm position by applying an equilibrium force on the joint. Furthermore, we controlled the vibrations of the arm by controlling the MR brake using an evaluation function. By this impedance control, response of the arm was improved without large loss of power.