A method for manufacturing an electric car battery cooling device including an upper case and a lower case formed of a clad plate, in which the upper case includes an upper case body formed of aluminum and an upper adhesive layer laminated on the upper case body, the lower case is coupled to the upper case and includes a lower case body formed of aluminum and a lower adhesive layer laminated on the lower case body, and an accommodation space is formed between the upper case and the lower case, the method comprising: a base material preparation step (ST-110) of preparing the upper case and the lower case, which are formed of the clad plate characterized by a flux application step (ST-120) of applying a flux on bonding surfaces of the upper case and the lower case such that the flux is laminated on the upper adhesive layer and the lower adhesive layer; and a thermal bonding step (ST-130) of stacking and heating the upper case and the lower case such that applied flux layers are in contact with each other wherein the flux includes a fluoride-based aluminum flux, which is mixed with water, and is prepared by mixing 15% by weight to 40% by weight of an aluminum flux with 60% by weight to 85% by weight of water, wherein the flux application step (ST-120) includes a first application step (ST-121) of applying the flux to the upper adhesive layer of the upper case and the lower adhesive layer of the lower case, and a second application step for applying the flux to the applied flux layer again, and wherein the method further comprises a flux drying step (ST-122) of drying the flux layer applied in the first application step (ST-121), which is performed between the first application step (ST-121) and the second application step (ST-123) such that a greater amount of fluxes penetrates into uncoated portions or pores generated in the flux layer which is previously applied, thereby forming the flux layer having a more dense structure. a base material preparation step (ST-110) of preparing the upper case and the lower case, which are formed of the clad plate characterized by a flux application step (ST-120) of applying a flux on bonding surfaces of the upper case and the lower case such that the flux is laminated on the upper adhesive layer and the lower adhesive layer; and a thermal bonding step (ST-130) of stacking and heating the upper case and the lower case such that applied flux layers are in contact with each other wherein the flux includes a fluoride-based aluminum flux, which is mixed with water, and is prepared by mixing 15% by weight to 40% by weight of an aluminum flux with 60% by weight to 85% by weight of water, wherein the flux application step (ST-120) includes a first application step (ST-121) of applying the flux to the upper adhesive layer of the upper case and the lower adhesive layer of the lower case, and a second application step for applying the flux to the applied flux layer again, and wherein the method further comprises a flux drying step (ST-122) of drying the flux layer applied in the first application step (ST-121), which is performed between the first application step (ST-121) and the second application step (ST-123) such that a greater amount of fluxes penetrates into uncoated portions or pores generated in the flux layer which is previously applied, thereby forming the flux layer having a more dense structure.