The oil cooler structure of an automatic transmission includes an upper tube plate reinforcement and a lower tube plate reinforcement. Each tube plate reinforcement is arranged in a layer in an oil passage formed between a plane of an upper tube plate and a plane of a lower tube plate. Each tube plate reinforcement is also respectively welded to the upper tube plate and the lower tube plate and embossed parts thereof protruding toward the center of the oil passage being mutually abutted and welded therebetween. Accordingly, the assembled rigidity of the upper and lower tube plates is improved, and an entire durability of an oil cooler is enhanced for improved quality.