In order to seal air leak from lung dissection plane in thoracotomy, we studied diode laser irradiation (wavelength: 810 nm) with surface stain of indocyanine green (ICG, peak absorption wavelength: 805 nm) ex vivo. In general, this air leak is sealed by suturing with weak tension and large margin of parenchyma. This suturing requires surgeon's skill and takes long time. Moreover, lung ventilatory performance is significantly impaired. Since laser tissue welding is novel method to adhere living tissue with thin thermally denatured attachment layer, we propose to seal the lung dissection plane with laser irradiation. Our aim of this study is to investigate the sealing mechanism as well as optimum condition to develop reliable laser sealing method for dissected lung plane in surgery, using practical laser-dye combination. Compartment of extracted porcine lung was prepared as a lung model, which volume was approximately 60 cm3. ICG solution (2.5 mg/ml) was applied to the dissection plane of this lung model with 1minute. The diode laser (power density: 8-40 W/cm2) irradiated to the plane, moving the laser spot with constant speed (v = 1mm/s). The heat degeneration depth and smoothness of the laser irradiated surface were observed by a microscope. When power density was over 24 W/cm2, heat degeneration depth was over 1.5 × 10-4 m. There were no pin holes on the surface and the air leak seemed to be sealed completely. We also evaluated the air leak by endotracheal pressure. In the case of above condition, the heat degeneration depth was the same that of previous reported result with CO2 laser.