1. An ultrasound imaging device, comprising: a data acquiring unit configured to acquire 3-dimensional ultrasound image data based on receive signals formed based on ultrasound echoes reflected from a target object; a filtering unit configured to determine a size of a filtering mask of a filter, said size being adaptively determined according to an amount of the 3-dimensional ultrasound image data in data acquisition directions, the filtering unit being further configured to filter the 3-dimensional ultrasound image data by using the filtering mask having the determined size to reduce speckle noise; a scan converting unit configured to scan-convert the filtered 3-dimensional ultrasound image data; and a 3-dimensional rendering unit configured to perform 3-dimensional rendering upon the scan-converted 3-dimensional ultrasound image data to form a 3-dimensional ultrasound image, wherein the filtering unit is configured to determine the size of the filtering mask according to a data acquisition ratio of an amount of the 3-dimensional data in data acquisition directions, and wherein the size of the filtering mask is determined to have different lengths in at least two acquisition directions.