A semiconductor memory device and an operation method thereof that can accurately read setting information from a memory cell array when a power supply is turned on are provided. The flash memory includes a memory cell array, a detecting portion, a ROM and a control portion. The detecting portion detects that the power supply is turned on. The ROM stores at least a code for performing a reading operation of the memory cell array and stores a special code in a specific address. The control portion controls the reading of the ROM. When the detecting portion detects the power-on of the power supply, the control portion reads the special code from the ROM and determines whether the read special code is correct or not, reads the code if the determination is correct and again reads the special code if the determination is incorrect.