An ultrasound observation system (1) includes an ultrasound transducer (11) that generates an ultrasound signal by transmitting and receiving ultrasound to and from a subject, an automatic pressurizing mechanism (12) that applies a pressing force to the subject to cause pressurizing displacement, an elastic-image-generation displacement measuring circuit (33) that measures an imaging displacement amount of the subject based on the ultrasound signal, an elastic modulus calculation circuit (34) that calculates an elastic modulus of the subject based on the imaging displacement amount, a pressurizing-mechanism-control displacement measuring circuit (31) that measures a controlling displacement amount of the subject based on the ultrasound signal, and a pressurizing mechanism control circuit (32) that controls the automatic pressurizing mechanism (12) based on the controlling displacement amount, wherein the pressurizing mechanism control section (32) calculates a period at which the controlling displacement amount on a basis of the spontaneous displacement of the subject is equal to or greater than a preset threshold, and controls the automatic pressurizing mechanism (12) to apply the pressing force at timing in accordance with the calculated period.
