In Denso, the resin (engineering plastic) of automobile parts has been focused as the lightweighting in oreder to protect global environment now. One of the problems in the lightweighting of engineering plastic PBT, had improvement of the adhesive strength. To solve the problem, we analized the PBT surface in the nano-meter molecular size by SAICAS, SVM and GIXD in SPring-8. As the result, we found that the cause of low adhesive strength of PBT is WBL (Weak Boundary Layer) after annealing. As the measurement of PBT adhesive strength improvement we developed a plasma surface treatment etc. based on the mechanism and adopted this technology in automobile resin parts and contributed to the lightweighting/ reliability improvement of the automobile.