An ultrasound system comprising: a storage that stores image data of a living body containing a target object, which is acquired before undergoing radiofrequency ablation RFA; an ultrasound probe that transmits an ultrasound signal to the living body during the RFA and receives an ultrasound echo signal reflected from the living body to create a reception signal; an ultrasound data acquirer configured to acquire ultrasound data by using the reception signal; and a processor that produces a reference image by using the image data and a plurality of ultrasound images by using the ultrasound data, performs image registration between the reference image and each of the plurality of ultrasound images, compares brightness values of the reference image and a brightness value of each of the plurality of ultrasound images that have undergone the image registration with each other, compares a preset threshold value and a brightness value difference between the reference image and each of the plurality of ultrasound images, detects an ultrasound image for producing change information about the target object from among the plurality of ultrasound images based on the comparison of the preset threshold value and the brightness value difference, and produces the change information indicative of a change in the target object based on the detection. a storage that stores image data of a living body containing a target object, which is acquired before undergoing radiofrequency ablation RFA; an ultrasound probe that transmits an ultrasound signal to the living body during the RFA and receives an ultrasound echo signal reflected from the living body to create a reception signal; an ultrasound data acquirer configured to acquire ultrasound data by using the reception signal; and a processor that produces a reference image by using the image data and a plurality of ultrasound images by using the ultrasound data, performs image registration between the reference image and each of the plurality of ultrasound images, compares brightness values of the reference image and a brightness value of each of the plurality of ultrasound images that have undergone the image registration with each other, compares a preset threshold value and a brightness value difference between the reference image and each of the plurality of ultrasound images, detects an ultrasound image for producing change information about the target object from among the plurality of ultrasound images based on the comparison of the preset threshold value and the brightness value difference, and produces the change information indicative of a change in the target object based on the detection. produces a reference image by using the image data and a plurality of ultrasound images by using the ultrasound data, performs image registration between the reference image and each of the plurality of ultrasound images, compares brightness values of the reference image and a brightness value of each of the plurality of ultrasound images that have undergone the image registration with each other, compares a preset threshold value and a brightness value difference between the reference image and each of the plurality of ultrasound images, detects an ultrasound image for producing change information about the target object from among the plurality of ultrasound images based on the comparison of the preset threshold value and the brightness value difference, and produces the change information indicative of a change in the target object based on the detection.