In a power supply device, the controller outputs a control signal specifying a voltage value. The voltage conversion unit converts a first voltage to a second voltage in response to a control signal specifying a voltage value output from the controller. The voltage conversion unit converts the first voltage to a start voltage, as the second voltage, in response to a first control signal specifying a start voltage value output from the controller and further converts the first voltage to a target voltage, as the second voltage, in response to a second control signal specifying a target voltage value output from the controller. A transition period of time is intervened between generation of the start voltage and generation of the target voltage during which the controller outputs a third control signal specifying the intermediate voltage value between the start voltage value and the target voltage value.