An ultrasound endoscope system comprising: an ultrasound endoscope for elastography comprising: a long and flexible insertion portion configured to be inserted into a subject; a distal end portion arranged at a distal end of the insertion portion; an ultrasound observation portion arranged at the distal end portion; an opening portion (22, 23, 24, 80b, 80d, 80e) enabling fluid to be ejected from a vicinity of the ultrasound observation portion in a projecting direction of the ultrasound observation portion for pressing living tissue; a fluid conduit (47, 59) communicating with the opening portion (22, 23, 24, 80b, 80d, 80e); and a flow rate adjusting portion configured to cause the ejection of the fluid to periodically change by switching a flow rate of the fluid flowing through the fluid conduit (47, 59) between two states at an arbitrary duty ratio, characterized in that the ultrasound endoscope system further comprises a pressing control portion configured to control the flow rate through the flow rate adjusting portion so that the flow rate changes at a predetermined duty ratio; a probe control portion (121, 122, 123, 124,125) configured to drive and control the ultrasound observation portion in two pressing states controlled by the pressing control portion to acquire an ultrasound signal of the living tissue; and an elastographic image generating portion (126,127,128,129) configured to measure an amount of displacement of the subject based on the ultrasound signal and to generate an elastographic image showing distribution of moduli of elasticity for respective coordinates of the subject based on the amount of displacement. an ultrasound endoscope for elastography comprising: a long and flexible insertion portion configured to be inserted into a subject; a distal end portion arranged at a distal end of the insertion portion; an ultrasound observation portion arranged at the distal end portion; an opening portion (22, 23, 24, 80b, 80d, 80e) enabling fluid to be ejected from a vicinity of the ultrasound observation portion in a projecting direction of the ultrasound observation portion for pressing living tissue; a fluid conduit (47, 59) communicating with the opening portion (22, 23, 24, 80b, 80d, 80e); and a flow rate adjusting portion configured to cause the ejection of the fluid to periodically change by switching a flow rate of the fluid flowing through the fluid conduit (47, 59) between two states at an arbitrary duty ratio, a long and flexible insertion portion configured to be inserted into a subject; a distal end portion arranged at a distal end of the insertion portion; an ultrasound observation portion arranged at the distal end portion; an opening portion (22, 23, 24, 80b, 80d, 80e) enabling fluid to be ejected from a vicinity of the ultrasound observation portion in a projecting direction of the ultrasound observation portion for pressing living tissue; a fluid conduit (47, 59) communicating with the opening portion (22, 23, 24, 80b, 80d, 80e); and a flow rate adjusting portion configured to cause the ejection of the fluid to periodically change by switching a flow rate of the fluid flowing through the fluid conduit (47, 59) between two states at an arbitrary duty ratio, characterized in that the ultrasound endoscope system further comprises a pressing control portion configured to control the flow rate through the flow rate adjusting portion so that the flow rate changes at a predetermined duty ratio; a probe control portion (121, 122, 123, 124,125) configured to drive and control the ultrasound observation portion in two pressing states controlled by the pressing control portion to acquire an ultrasound signal of the living tissue; and an elastographic image generating portion (126,127,128,129) configured to measure an amount of displacement of the subject based on the ultrasound signal and to generate an elastographic image showing distribution of moduli of elasticity for respective coordinates of the subject based on the amount of displacement.