Nonvolatile storage device and data writing method thereof

A nonvolatile storage device according to the invention is a nonvolatile storage device into which data is inputted from an external device on a sector unit, and includes: a main memory which is nonvolatile and in which data is written on a page unit, the page unit being larger than the sector unit; an auxiliary memory which holds at least a single page worth of the input data; a memory judging unit that judges whether or not data held in the auxiliary memory is equal to or larger than data of the page unit; and a memory control unit that writes, in a new page of the main memory on the page unit, the data held in the auxiliary memory when the memory judgment unit judges that the data held in the auxiliary memory is equal to or larger than data of the page unit.