An engine starting device for starting up an engine, the engine starting device comprising: a high-voltage power storage unit; a DC/DC converter electrically connected to the high-voltage power storage unit; a first switch having a first end and a second end, the first end of the first switch being electrically connected to the DC/DC converter; a second switch having a first end and a second end, the first end of the second switch being electrically connected to the second end of the first switch at a node; a low-voltage power storage unit electrically connected to the second end of the second switch, the low-voltage power storage unit having a lower voltage than the high-voltage power storage unit; a starter electrically connected to the node for starting up the engine; and a controller electrically connected to the DC/DC converter, the first switch, and the second switch, wherein the controller is configured to: for starting up the starter, control the DC/DC converter to allow the DC/DC converter to output electric power of the high-voltage power storage unit to the starter and to start rotating the starter at a certain time point by turning on the first switch while turning off the second switch, when a predetermined duration elapses from the certain time point or a rotation speed of the starter reaches a predetermined rotation speed, control the second switch to supply electric power of the low-voltage power storage unit to the starter together with the electric power of the high-voltage power storage unit by turning on the second switch; and turn off the first switch and the second switch when the engine completely starts up, characterized in that the controller is further configured to increase an output voltage of the DC/DC converter with respect to time when the first switch is turned on while the second switch is turned off. a high-voltage power storage unit; a DC/DC converter electrically connected to the high-voltage power storage unit; a first switch having a first end and a second end, the first end of the first switch being electrically connected to the DC/DC converter; a second switch having a first end and a second end, the first end of the second switch being electrically connected to the second end of the first switch at a node; a low-voltage power storage unit electrically connected to the second end of the second switch, the low-voltage power storage unit having a lower voltage than the high-voltage power storage unit; a starter electrically connected to the node for starting up the engine; and a controller electrically connected to the DC/DC converter, the first switch, and the second switch, wherein the controller is configured to: for starting up the starter, control the DC/DC converter to allow the DC/DC converter to output electric power of the high-voltage power storage unit to the starter and to start rotating the starter at a certain time point by turning on the first switch while turning off the second switch, when a predetermined duration elapses from the certain time point or a rotation speed of the starter reaches a predetermined rotation speed, control the second switch to supply electric power of the low-voltage power storage unit to the starter together with the electric power of the high-voltage power storage unit by turning on the second switch; and turn off the first switch and the second switch when the engine completely starts up, for starting up the starter, control the DC/DC converter to allow the DC/DC converter to output electric power of the high-voltage power storage unit to the starter and to start rotating the starter at a certain time point by turning on the first switch while turning off the second switch, when a predetermined duration elapses from the certain time point or a rotation speed of the starter reaches a predetermined rotation speed, control the second switch to supply electric power of the low-voltage power storage unit to the starter together with the electric power of the high-voltage power storage unit by turning on the second switch; and turn off the first switch and the second switch when the engine completely starts up,