This paper proposes a plug-in feedback scheme for vibration-suppression control of a serial two-link robot arm with joint elasticity due to the Harmonic-drive gear. The serial two-link arm simulates the 1st and 2nd joints of the SCARA (Selective Compliance Assembly Robot Arm)-type robot or the 2nd and 3rd joints of the PUMA (Programmable Universal Manipulator Arm)-type robot. In order to suppress the arm-tip vibration of both robot types, it is important to control the basic two-link arm. We propose a torsion-angular velocity feedback (TVFB) scheme, which can be plugged into existing joint servos (PI velocity controllers), using a nonlinear state-observer based on a physically parameterized dynamic model of the serial two-link robot arm. Physical parameters of the elastic-joint model are accurately estimated by the 'decoupling identification method' previously proposed by the authors. The feedback gains of the observer are set identical to the PI gains tuned for the existing joint servos. Thus the nonlinear observer, which estimates the torsion-angular velocity, is designless. Also, simple gain-scheduling scheme with few hand-tuned state-feedback gains is implemented for the TVFB, taking the arm-posture and payload changes into consideration. We just have to manually tune the few state-feedback gains. Several experiments are conducted to demonstrate the effectiveness of the TVFB using the serial two-link robot arm.