An ultrasound observation system for transmitting ultrasound to a subject, receiving the ultrasound reflected by the subject, and generating an image based on an ultrasound signal obtained by the received ultrasound, the ultrasound observation system comprising: an ultrasound transducer that transmits the ultrasound to the subject, receives the ultrasound reflected by the subject and generates the ultrasound signal from the received ultrasound; a pressurizing section that applies a pressing force to the subject to cause pressurizing displacement; a displacement-related-amount detecting section (11; 13) that detects a displacement related amount relating to displacement of the subject; an elastic-image displacement measuring section that measures an imaging displacement amount of the subject based on the ultrasound signal so as to generate an elastic image of the subject; an elastic modulus calculation section that calculates an elastic modulus of the subject based on the imaging displacement amount; a pressurizing-control displacement measuring section that measures a controlling displacement amount of the subject based on the displacement related amount so as to control the pressurizing section; and a pressurizing control section that controls the pressurizing section based on the controlling displacement amount, characterized in that the pressurizing control section estimates the controlling displacement amount on a basis of the pressurizing displacement from timing at which the pressurizing section applies the pressing force, and calculates a period on a basis of spontaneous displacement of the subject by calculating a period at which the controlling displacement amount with the estimated controlling displacement amount excluded is equal to or greater than a preset threshold. an ultrasound transducer that transmits the ultrasound to the subject, receives the ultrasound reflected by the subject and generates the ultrasound signal from the received ultrasound; a pressurizing section that applies a pressing force to the subject to cause pressurizing displacement; a displacement-related-amount detecting section (11; 13) that detects a displacement related amount relating to displacement of the subject; an elastic-image displacement measuring section that measures an imaging displacement amount of the subject based on the ultrasound signal so as to generate an elastic image of the subject; an elastic modulus calculation section that calculates an elastic modulus of the subject based on the imaging displacement amount; a pressurizing-control displacement measuring section that measures a controlling displacement amount of the subject based on the displacement related amount so as to control the pressurizing section; and a pressurizing control section that controls the pressurizing section based on the controlling displacement amount, characterized in that the pressurizing control section estimates the controlling displacement amount on a basis of the pressurizing displacement from timing at which the pressurizing section applies the pressing force, and calculates a period on a basis of spontaneous displacement of the subject by calculating a period at which the controlling displacement amount with the estimated controlling displacement amount excluded is equal to or greater than a preset threshold.