Purpose <p>Bronchoscopic tracheal intubation is an indispensable technique in patients with difficult airway, but it is often difficult, particularly for less-experienced anesthesiologists, to obtain a view of the glottis by bronchoscopy. We hypothesized that insertion of a flexible bronchoscope to a predetermined depth would facilitate bronchoscopic intubation.</p> Methods <p>Fifty anesthetized patients were randomly allocated to two groups, and inexperienced anesthesiologists either inserted a bronchoscope along the center of the tongue (in group C), or along the hard palate to a predetermined depth based on the mouth-to-tragus distance (in group S). The success rate of obtaining a bronchoscopic view of the glottis (primary outcome) and other factors were compared.</p> Results <p>The success rate of obtaining a bronchoscopic view of the glottis was significantly higher in group S (24 of 25 patients) than in group C (17 of 25 patients) (<i>P</i> = 0.02; 95%CI for difference: 8 – 48%]). The success rate of tracheal intubation was significantly higher in group S (18 of 25 patients) than in group C (8 of 25) (<i>P</i> = 0.01). Time to glottic visualization was significantly shorter in group S (24 [14 – 40] s) than in group C (49 [30 – 120]] s) (<i>P</i> = 0.003), and apnea time was significantly shorter in group <i>S</i> (81 [52 – 120] s) than in group <i>C</i> (120 [109 – 120] s) (<i>P</i> = 0.004).</p> Conclusions <p>Insertion of a bronchoscope for the mouth-to-tragus distance would increase the success rate of bronchoscopic intubation, by facilitating to obtain a bronchoscopic view of the glottis.</p> Clinical trial registration <p>jRCT1032250593</p>