A compression device for compressing a subject used in ultrasonic measurement that receives ultrasonic waves from a subject to acquire biological information of the subject, comprises: two compression plates used when compressing the subject, and face each other in a vertical direction with respect to a horizontal plane; a flexible member arranged so as to cover and seal an area between both side surfaces of and a bottom surface between the two compression plates, and so as not cover an area between top surfaces of the two compression plates such that an upper portion is open; a compression mechanism that relatively changes a distance between the two compression plates to sandwich and compress the subject; and a piping unit that supplies a matching liquid to a space portion surrounded by the two compression plates and the flexible member, and discharges the matching liquid that has been supplied.