This paper presents a self-sensing method for semi-active vibration suppression that measures only the value of piezoelectric voltage. This self-sensing method is implemented with a Kalman filter with extended system equations, instead of the conventional bridge-circuit technique. The method has several advantages over the bridge-circuit self-sensing method, such as being applicable to MIMO systems. Experiments showed that our selfsensing system suppressed vibrations by combining the state-switching control and the synchronized-switching control. We confirmed that the self-sensing method is robust against model errors through the experiment with intentional frequency shift.