A method of conducting an endoscopic observation of an inner wall of a gastrointestine by applying a diagnostic agent to the inner wall of the gastrointestine, the diagnostic agent being an acidic aqueous solution of pH 1 to 5 containing a colorant, and conducting the endoscopic observation of the inner wall. The acidic aqueous solution can contain at least one acid selected from a carboxylic acid, hydrochloric acid, sulfuric acid and phosphoric acid. By applying the diagnostic agent to the inner wall of the gastrointestine in the endoscopic observation, it is possible to clearly observe a lesion which is difficult to be determined. In particular, the method can be used for observing lesions having cancer cells in the stomach or the Barrett's esophagus.