A nasal mask is adapted to attached on a face of a subject so as to cover a nose of the subject. A shell has a contact edge adapted to be brought into close contact with the face of the subject, thereby forming a sealed space therein. An attachment portion is disposed in the shell and has an inner space. The attachment portion is configured to detachably engage with a sensor disposed outside the shell and operable to measure concentration of carbon dioxide gas expired from the nostrils. A guide member is disposed inside the shell and forming a passage adapted to introduce expired gas from the nostrils into the inner space. The shell is formed with an inlet adapted to introduce external gas into the sealed space. The expired gas is guarded by the guide member from the external gas introduced through the inlet. The attachment portion is formed with an outlet adapted to discharge the expired gas from the inner space to the sealed space.