To provide a mask capable of reducing a load of a patient while suppressing lowering of accuracy at which a subject's exhaled air is measured. A mask to be put on a subject's face includes a mask body portion demarcating an internal space in a state of covering part of the subject's face and a cup-shaped nasal cup covering a subject's nose in a state of being arranged inside the internal space, in which the nasal cup includes a first wall portion covering the subject's nose, a second wall portion arranged under the subject's nostrils, and an exhaled air discharge portion guiding an exhaled air from the subject's nose to an exhaled air sensor, and at least part of the exhaled air from the subject's nose is guided toward the exhaled air discharge portion by the second guide portion in a state where the nasal cup covers the subject's nose.
