The importance of the middle meatus-anterior ethmoid complex, or ostiomeatal complex(O-M complex or O-M unit) in inflammatory sinus disease was well recognized by early physicians. Caldwell, Hajek and Zuckerkandl called attention to the close relationship between the frontal and maxillary sinus ostia and the anterior ethmoid cells. However, both anterior rhinoscopy and plain radiographs give us very limited information about the key area of the ostiomeatal complex. In addition, the symptoms of ethmoid sinus disease may be mild or referred, while frontal and maxillary sinus infections usually cause localized pain and discomfort. Recently the use of diagnostic nasal endoscopy and computed tomography(CT) have once again highlighted the importance of ostiomeatal disease and anatomic deformities of middle meatus in the pathogenesis of sinusitis. The nasal endoscopy and CT of the paranasal sinuses were essential in the diagnosis of the paranasal sinus diseases. Authors present the detail of diagnostic nasal endoscopic method with CT of the PNS for evaluating the PNS diseases.