MRAM and data writing method therefor

In an MRAM having main and sub-structures, selecting transistors are arranged so as to meet the arrangement order of main word lines, sub-word lines and the selecting transistors. The selecting transistor is driven to cause a snap back phenomenon to occur. As a result, data can be written to a memory cell using a substrate current, not a channel current. Moreover, a data may be written into a selected memory cell by discharge the charge which is charged in the main and sub word lines corresponding to the memory cell.