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 (ST123) 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 (ST123) 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. <EOS>