This paper describes a novel method to estimate varying periods from noise corrupted quasi- periodic bio- signals. Here an observer based on the biological mathematical model named Lotka- Volterra prey- predator equations is presented. The equation can express oscillations with different periods under constant coefficients in the equations. Mathematical properties of the model are summarized and a guideline of how to build a biological model for estimating the periods is shown. Simulations to estimate the periods for the heart- beat signals corrupted by noises were carried out and the accurate periods of the signals were estimated. The periods estimated by the proposed observer from the noisy heart- beat measured by the pneumatic method showed better results than those by the conventional method.