Background; Pleural micro-metastasis of lung cancer is detected by touch print cytology or pleural lavage cytology, but its prognostic impact has not elucidated yet. We hypothesize that recurrence may depend on the amount of tumor cells disseminated in pleural cavity, if the invasiveness of all cancer is the same. To predict the amount of tumor cells disseminated in pleural cavity, we need pleural surface area, distributed pattern of cells and concentration of cells per unit area. Human pleural surface area has not reported yet. In this report, we calculate the normal human pleural surface area using CT image data processing. Methods; Twenty persons were checked CT scan, and we obtained the data from each image. In order to calculate the pleural surface, the outline of lung was firstly extruded from CT image data using home-made Digitizer program. And the distance between CT images was calculated from the extruded outline. Finally a normal human pleural surface was calculated from function between the distance of consecutive CT images and the calculated length. Results; Their mean age is $65{pm}12$ years old (range $26{sim}77$), body weight is $62{pm}9;kg;(48{sim}80)$, and height is $167{pm}6;cm;(156{sim}176)$. The number of images used is $36{pm}7;(24{sim}51)$. Pleural surface area is $211,888{pm}35,756;mm^2;(143,880{sim}279,576)$. Right-side pleural surface area is $107,932;mm^2$ and Lt is $103,955;mm^2$. Costal, mediastinal and diaphragmatic surfaces of right-side pleura are $77,483;mm^2,;39,057;mm^2,;and;8,608;mm^2$ respectively, and left-side are $72,497;mm^2,;35,578;mm^2,;and;4,120;mm^2$ respectively. Conclusion; Normal human pleural surface area is calculated using CT image data at first and the result is about $0.212;m^2$.