A battery cooling system for cooling an in-vehicle battery for supplying electric energy to a vehicle, the battery cooling system being formed in a plate shape that extends in XY plane, the battery cooling system comprising: a cold plate which includes an inlet , a flow path , and an outlet of a coolant and configured to flow the coolant by a pump through the inlet, the flow path, and the outlet to transfer heat; wherein the flow path is provided inside the cold plate and the inlet and the outlet are provided on a front surface of the cold plate , a heat transfer plate which includes a plurality of contact portions which are configured to be in contact with the bottom surface of the in-vehicle battery via electrically insulating layer, wherein the contact portion are arranged at intervals in the X and Y direction wherein the heat transfer plate is formed in a rectangular plate shape that is longer in Y direction than in the X direction; and a plurality of heat pipes which transfer heat between the cold plate (13 and the contact portion , wherein the cold plate is positioned in a recessed portion extending in the Y direction at the center portion in the X direction of the heat transfer plate , wherein each heat pipe uses working fluid and metal container configuration, wherein a portion of each heat pipe in the vicinity of the cold plate is condensing portion and other portions of the heat pipes are evaporating portions, wherein a plurality of grooved portions extending in the X direction are formed on the heat transfer plate , the plurality of grooved portions being positioned between the contact portions and arranged at intervals in the Y direction, wherein the heat pipes are fitted in the grooved portions , each grooved portions being recessed inwards from the contact portions , wherein claw portions that protrude from each edge of the grooved portions toward inside of the grooved portions are formed on the heat transfer plate . a cold plate which includes an inlet , a flow path , and an outlet of a coolant and configured to flow the coolant by a pump through the inlet, the flow path, and the outlet to transfer heat; wherein the flow path is provided inside the cold plate and the inlet and the outlet are provided on a front surface of the cold plate , a heat transfer plate which includes a plurality of contact portions which are configured to be in contact with the bottom surface of the in-vehicle battery via electrically insulating layer, wherein the contact portion are arranged at intervals in the X and Y direction wherein the heat transfer plate is formed in a rectangular plate shape that is longer in Y direction than in the X direction; and a plurality of heat pipes which transfer heat between the cold plate (13 and the contact portion , wherein the cold plate is positioned in a recessed portion extending in the Y direction at the center portion in the X direction of the heat transfer plate , wherein each heat pipe uses working fluid and metal container configuration, wherein a portion of each heat pipe in the vicinity of the cold plate is condensing portion and other portions of the heat pipes are evaporating portions, wherein a plurality of grooved portions extending in the X direction are formed on the heat transfer plate , the plurality of grooved portions being positioned between the contact portions and arranged at intervals in the Y direction, wherein the heat pipes are fitted in the grooved portions , each grooved portions being recessed inwards from the contact portions , wherein claw portions that protrude from each edge of the grooved portions toward inside of the grooved portions are formed on the heat transfer plate . <EOS>