Motivated by the idea of the time delay estimation used in the Time Delay Control, Time Delay Observer (TDO) has been constructed. Owing to this idea, TDO does not require a model, and hence is easy to design for nonlinear systems in the phase variable form, even when the plant dynamics is not well known. Not including a model, TDO turned out to be robust to parameter variations and computationally efficient; yet achieved better accuracy. For this observer the convergence property and the sensitivity to sensor noise have been analyzed. Then, in a simulation with a nonlinear plant, and in an experiment with a robot, TDO achieved an accuracy level similar to sliding observer and better than a Kalman filter. In an experiment on a pneumatic cylinder control, TDO reconstructed states much better than a numerical differentiator with low-pass filtering.