A semiconductor storage device restraining the variation in threshold voltage of a memory unit is provided. The steps of the programming method for a flash memory include: setting a bit line to a program voltage or a program-protection voltage; applying a program pulse to the selected page; and verifying the programming of the selected page. Also, the steps further include: when the verification result indicates that there is a failed-shift memory cell which was passed previously but is failed presently, setting the voltage of the bit line of the failed shift memory to a mitigation voltage for mitigating the voltage of the next program pulse.