A method for sampling breath gas, includes collecting a first breath sample in a first bag. The first breath sample is an initial part of expired gas expired after inspiration. Additionally, the method includes collecting a second breath sample in a second bag. The second breath sample is a latter part of the expired gas. The method includes subtracting first mass spectral data obtained by mass spectroscopy of the first breath sample collected in the first bag from second mass spectral data obtained by mass spectroscopy of the second breath sample collected in the second bag.