According to the present invention, ultrasonic image quality can be improved by removing side lobe signals using a nonlinear filter that subtracts the side lobe signals calculated from a summed signal resulting from the received channel signals obtained by focusing signals received via an array transducer having a plurality of receiving elements and uses the calculated magnitude of the side lobe signals as a filter coefficient. The present invention includes an array transducer to receive an ultrasonic signal reflected from an imaging point and to output the reflected ultrasonic signal as a channel signal of a corresponding receiving element, a focusing delay module to temporally align the channel signals of the receiving elements, a summation unit to sum the temporally aligned channel signals and output the summed signal in order to form an ultrasonic image, a side lobe computation module to calculate a waveform of a side lobe signal generated due to a leakage of the ultrasonic signal, and a filter unit to filter the summed signal of the summation unit based on the magnitude of the side lobe signal computed by the side lobe computation module in order to improve ultrasonic image quality.
