A computer having a CPU which operates with at least two operation modes, comprising: a mode signal output unit outputting an operation mode signal corresponding to the operation mode of the CPU; a CPU power supply supplying power having a voltage level corresponding to the operation mode signal outputted from the mode signal output unit to the CPU; and a control unit controlling the CPU power supply to decrease an equivalent series resistance value to power outputted from the CPU power supply for a predetermined period of time for delay from when switching the operation mode of the CPU has been sensed, based on the operation mode signal outputted from the mode signal output unit.