Next, as illustrated in FIGS. 16A to 16C, the initial first semiconductor column AAi and the second insulating film 21 are etched to form the slit to divide the initial first semiconductor column AAi at the substantially center thereof in the X direction by using a lithography technique and an etching technique. FIG. 16B illustrates a cross section taken along line K-K in FIG. 16A, and FIG. 16C illustrates a cross section taken along line L-L in FIG. 16A. The gate insulating film 203 is formed on the inner wall of the slit, and a material for the conductive layer BW is further buried inside the gate insulating film 203. Accordingly, the conductive layer BW is formed to divide the initial first semiconductor column AAi at the substantially center thereof in the X direction. The conductive layer BW is electrically insulated from the first semiconductor columns AA1 and AA2 by the gate insulating film 203. Therefore, as illustrated in FIG. 16A, in the planar shape of the hole SH2, the initial first semiconductor column AAi is divided so that the conductive layer BW passes the substantially center of the initial first semiconductor column AAi. It is noted that it is preferable that the left and right areas of the initial first semiconductor column AAi after the division are substantially equal.
