Vibration energy harvesting extracts electrical energy from vibrating structures. The past studies of vibration energy harvesting suggest that the efficiency can be improved by switch regulation in the harvesting circuit. The switch-regulation is carried out depending on the motion of the target structure with the use of vibration sensors such as displacement sensor or accelerometer. This paper proposes a new vibration self-sensing method for switching energy harvesters that do not use those vibration sensors. In this method, the voltage of the piezoelectric transducer is measured, and the structural vibrational status is estimated from the measured voltage. The transducer voltage is not smooth and does not maintain the sinusoidal wave even when the structure vibrates in a sinusoidal wave because the switch energy harvesting method inverses the transducer voltage at every period. Thus, we establish a state observer based on a Kalman filter to estimate three state values of the target harvesting system: modal displacement, modal velocity, and electric charge in the transducer. This paper describes the construction processes for the observer. The observed value is the transducer voltage. We also show an electric circuit for measuring the transducer voltage. Finally, we confirm the efficiency of the proposed state observer for switch harvesting with numerical simulations.