A power supply device in a semiconductor memory includes a power control means and a power generation means. The power control means divides a self-refresh section into an active-precharge mode and an idle mode depending on an operation characteristic of the semiconductor memory, and generates a control signal for controlling power strength applied to the semiconductor memory during operation in each mode. The power generation mode generates a different power level in response to a power control signal from the power control means to provide to the semiconductor memory. Meanwhile, the power supply device according to the present invention provides relatively strong power to the semiconductor memory a predetermined time period in advance of the active-precharge mode.