1. A writing control method of sequentially writing data into a semiconductor storage device, the semiconductor storage device including a memory system having: (a) a memory array having a plurality of memory elements each having: a gate electrode provided on a semiconductor layer with an intervening gate insulating film; a channel region provided beneath the gate electrode; a diffusion region provided on both sides of the channel region, having an opposite polarity to the channel region; and a memory functioning member, provided on both sides of the gate electrode, having a function of holding electric charges; (b) a page buffer circuit including a first plane and a second plane; and (c) a memory control circuit controlling writing with respect to the memory array and being capable of accessing the page buffer circuit, the writing control method comprising: a first step of writing first data into the first plane; a second step of controlling the memory control circuit to write the first data having been written into the first plane into the memory array; a third step of writing second data into the second plane while the first data having been written into the first plane is written into the memory array; and a fourth step of controlling the memory control circuit to write the second data having been written into the second plane into the memory array.