A battery cooling system (10A, 10B, 10C) 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 (10A, 10B, 10C) 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.