An air-pad sensor (APS) was developed to record respiratory effort non-invasively. The APS is placed under the subjects' bodies and acoustic signals received by the pad are analysed. Nine patients with obstructive sleep apnea (OSA) and two healthy subjects were examined using an APS and daytime polysomnography, including intraesophageal pressure (Peso), simultaneously. The agreement ratio in terms of the central apnea time between the APS and the Peso was 93.4%, although the ratio between the Peso and an abdominal strain gauge was 40.7%. The agreement ratio in terms of respiratory rate among the Peso, APS, and abdominal strain gauge in patients with OSA was high; but the agreement ratio was low (74.4%) between the Peso and the strain gauge in mixed apnea.