Pulmonary function is generally evaluated based on the overall capacity of both lungs; this evaluation is performed by a pulmonary functional test using a spirometer. Diaphragm movement has a direct association with pulmonary function. Therefore, evaluation of diaphragm motion is also helpful for estimating pulmonary function in the lung unit. The purpose of this study was to investigate the utility of dynamic analysis of the diaphragm using dynamic flat-panel detector (FPD) imaging for pulmonary function assessment in the lung unit. Sequential chest radiographs of four pigs (body weight approximately 20−30 kg) were obtained using a dynamic FPD system under respiratory control with a ventilator (100, 200, 300, 400, and 500 mL). Diaphragm excursion was measured and then analyzed the correlation with inspired volume. We also created porcine models of atelectasis by a catheter procedure and investigated whether lungs affected by atelectasis could be detected as reduced diaphragm excursion. To facilitate visual evaluation, temporal cross-sectional images were created, with the x-axis representing time, using a linear interpolation method. There was a strong correlation between inspired volume and diaphragm excursion (r = 0.96). In porcine models of atelectasis, diaphragm movement of an affected lung was restricted and was reduced, on average, to 44% of that in unaffected lungs. Reduction in diaphragm movement was also observed in the temporal cross-sectional images. Dynamic FPD imaging allows for relative pulmonary function assessment based on diaphragm movement, and unilateral abnormalities could be detected as reduced diaphragm excursion, even with a normal inspired volume.