Overnight polysomnography (PSG) is indispensable for diagnosis of obstructive sleep apnea hypopnea syndrome. However, studies on interscorer agreement on PSG scoring between laboratories are few. The purpose of this study was to examine the reliability of interscorer agreement on PSG scoring among 16 sleep laboratories in Japan. We found a relatively moderate interscorer reliability of the index of oxygen desaturation and arousal during sleep, but a relatively low reliability of the index of transient reduction in and complete cessation of breathing (apnea hypopnea index). The median rate of interscorer coincidence of sleep staging was the lowest for slow wave (deep) sleep (23.5%), followed by those for Stage 1 (59.8%), Wake (73.2%) and Stage 2 (74.2%) in this order, and rapid eye movement was the most reliably identified stage (91.3%). The median rate of interscorer coincidence for all stages was 71.8%. The present study demonstrates that scorers tend to analyze PSG data according to a relatively empirical decision as opposed to a rule-dependent decision. Further detailed scoring manuals are required to decrease the interscorer discrepancy in PSG scoring.