An apparatus and method capable of producing a tissue Doppler image without repeatedly transmitting ultrasound signals by the ensemble number to each scan line. An ultrasound beam is transmitted in a non-sequential manner and a plurality of receive beams received in response to each transmission of the transmit beam are grouped into an increment data group of a scan line index increasing direction and a decrement data group of a scan line index decreasing direction. Auto correlation is performed on the increment data group of the scan line index ascending order and decrement data group of the scan line index descending order, weights are applied to respective auto correlation values for the increment and decrement data group, and then the auto correlation values with the weight applied are summed to compute a mean phase. At least one of velocities, powers and variances in response to the mean phase is outputted to produce TDI.