A biological information acquiring system comprising: an airway adaptor (10,10A,10B) which is adapted to be attached to a face of a subject, a carbon dioxide sensor configured to detect a concentration of carbon dioxide contained in exhalation of the subject, and a temperature sensor (104,106), the airway adaptor comprising: an airway case which includes a gas passage, the airway case adapted to be attachable to a part of the carbon dioxide sensor, the exhalation flowing into the gas passage, the part of the carbon dioxide sensor including a first light-emitting element and a first light-receiving element; a nasal cannula (30,30A,30B) having a pair of insertion portions (31, 32) which are adapted to be inserted respectively into nostrils of the subject to guide exhalation from the nostrils to the gas passage when the airway adaptor is attached to the face; a mouth guide adapted to be placed in front of a mouth of the subject to guide exhalation from the mouth to the gas passage when the airway adaptor is attached to the face; a second light-emitting element supported by one of the pair of insertion portions; and a second light-receiving element supported by the other of the pair of insertion portions, wherein when the pair of insertion portions (31, 32) are inserted into the nostrils of the subject, the second light-emitting element and the second light-receiving element are opposed to each other across a nasal septum of the subject, characterised in that the second light-receiving element is configured to operate as a part of the temperature sensor (104,106). an airway adaptor (10,10A,10B) which is adapted to be attached to a face of a subject, a carbon dioxide sensor configured to detect a concentration of carbon dioxide contained in exhalation of the subject, and a temperature sensor (104,106), the airway adaptor comprising: an airway case which includes a gas passage, the airway case adapted to be attachable to a part of the carbon dioxide sensor, the exhalation flowing into the gas passage, the part of the carbon dioxide sensor including a first light-emitting element and a first light-receiving element; a nasal cannula (30,30A,30B) having a pair of insertion portions (31, 32) which are adapted to be inserted respectively into nostrils of the subject to guide exhalation from the nostrils to the gas passage when the airway adaptor is attached to the face; a mouth guide adapted to be placed in front of a mouth of the subject to guide exhalation from the mouth to the gas passage when the airway adaptor is attached to the face; a second light-emitting element supported by one of the pair of insertion portions; and a second light-receiving element supported by the other of the pair of insertion portions, wherein an airway case which includes a gas passage, the airway case adapted to be attachable to a part of the carbon dioxide sensor, the exhalation flowing into the gas passage, the part of the carbon dioxide sensor including a first light-emitting element and a first light-receiving element; a nasal cannula (30,30A,30B) having a pair of insertion portions (31, 32) which are adapted to be inserted respectively into nostrils of the subject to guide exhalation from the nostrils to the gas passage when the airway adaptor is attached to the face; a mouth guide adapted to be placed in front of a mouth of the subject to guide exhalation from the mouth to the gas passage when the airway adaptor is attached to the face; a second light-emitting element supported by one of the pair of insertion portions; and a second light-receiving element supported by the other of the pair of insertion portions, wherein when the pair of insertion portions (31, 32) are inserted into the nostrils of the subject, the second light-emitting element and the second light-receiving element are opposed to each other across a nasal septum of the subject, characterised in that the second light-receiving element is configured to operate as a part of the temperature sensor (104,106).