An ultrasound system, comprising: an ultrasound data acquisition unit configured to sequentially acquire ultrasound data corresponding to a living body; and a processing unit the processing unit comprising: an ultrasound image forming section configured to sequentially form a plurality of ultrasound images based on the ultrasound data; a determining section configured to compare a reference frame rate and a current frame rate to output a determining result based on an i th (i is a positive integer) ultrasound image and an i th frame average image; a coefficient estimating section configured to output the frame average coefficient for performing a frame average process based on the determining result; a frame average processing section configured to form an (i+1) th frame average image by performing the frame average process between the i th frame average image and an (i+1) th ultrasound image based on the frame average coefficient, characterized in that the coefficient estimating section comprises: a resampling section configured to resample the i th ultrasound image and the i th frame average image according to the current frame rate based on the determining result to form first resampling data and second resampling data; an estimating section configured to output the frame average coefficient based on the first resampling data and the second resampling data; a first multiplying section configured to multiply the frame average coefficient with the first resampling data to output first multiplying data; a delaying section configured to apply a delay to addition data; a second multiplying section configured to multiply the frame average coefficient with the delayed addition data to output second multiplying data; and an adding section configured to add the first multiplying data and the second multiplying data to output the addition data. an ultrasound data acquisition unit configured to sequentially acquire ultrasound data corresponding to a living body; and a processing unit the processing unit comprising: an ultrasound image forming section configured to sequentially form a plurality of ultrasound images based on the ultrasound data; a determining section configured to compare a reference frame rate and a current frame rate to output a determining result based on an i th (i is a positive integer) ultrasound image and an i th frame average image; a coefficient estimating section configured to output the frame average coefficient for performing a frame average process based on the determining result; a frame average processing section configured to form an (i+1) th frame average image by performing the frame average process between the i th frame average image and an (i+1) th ultrasound image based on the frame average coefficient, an ultrasound image forming section configured to sequentially form a plurality of ultrasound images based on the ultrasound data; a determining section configured to compare a reference frame rate and a current frame rate to output a determining result based on an i th (i is a positive integer) ultrasound image and an i th frame average image; a coefficient estimating section configured to output the frame average coefficient for performing a frame average process based on the determining result; a frame average processing section configured to form an (i+1) th frame average image by performing the frame average process between the i th frame average image and an (i+1) th ultrasound image based on the frame average coefficient, a resampling section configured to resample the i th ultrasound image and the i th frame average image according to the current frame rate based on the determining result to form first resampling data and second resampling data; an estimating section configured to output the frame average coefficient based on the first resampling data and the second resampling data; a first multiplying section configured to multiply the frame average coefficient with the first resampling data to output first multiplying data; a delaying section configured to apply a delay to addition data; a second multiplying section configured to multiply the frame average coefficient with the delayed addition data to output second multiplying data; and an adding section configured to add the first multiplying data and the second multiplying data to output the addition data.