An apparatus of supplying a fire extinguishing agent to a high voltage battery pack of a vehicle when a fire occurs in a battery case installing a high voltage battery pack includes a fire detection device that is configured to detect an occurrence of the fire in the battery pack, an inflow plug which is formed at the battery case and is configured to allow the fire extinguishing agent to flow into the battery case when open, a fire extinguishing agent supply pipe connected to the inflow plug and guiding the fire extinguishing agent to the inflow plug, and an inflow plug opening device that is configured to open the inflow plug when the fire is detected in the fire detection device.