A two-dimensional array including two-dimensionally arranged transducers with M rows (M=4) and N columns (N=12) is separated into eight sub-arrays each including six transducers with m rows (m=2) and n columns (n=3). The sub-arrays with J rows (J=2) and K columns (K=4) are connected to eight intra-group processors IP(JJ, KK) (JJ=1 to 2, KK=1 to 4), respectively. A switch allows the selection of j pieces in the row direction (j=2) (j≦J) and k pieces in the column direction (k