A computer system has a normal mode for performing its work in a normal and regular manner and a standby mode which progresses to a power saving mode after storing information about the state of the work in process in a memory. The computer system comprises: a power management controller generating a control signal with respect to the normal mode and the standby mode, and storing the power mode information about the respective modes; a power supply part supplying normal power in the normal mode and standby power in the standby mode depending upon the control signal of the power management controller; a standby mode storing portion storing information identifying the standby mode when the mode of the system is shifted to the standby mode; and a controller determining whether the standby mode identification information exists in the standby mode storing portion when the power mode information of the power management controller indicates the standby mode during the re-booting operation, and performing normal booting when no standby mode identification information exists in the standby mode storing portion. With this configuration, when power is cut-off from and then re-supplied to a computer system, a stable booting operation can be performed.