A mask configured to be attached to the face of a subject includes a wall section that defines an internal space and covers at least a portion of a nose and a mouth of the subject, an expired gas introduction section that is disposed in the internal space and introduce the subject's expired gas, and a communication section defining a communication channel through which the subject's expired gas introduced from the expired gas introduction section is introduced into an expired gas concentration detection sensor. The position of the expired gas introduction section in the internal space is variable.