An exhalation collected in a breath bag capable of expanding and contracting is sucked into a gas inlet vessel 21 with a given volume (Va), and then the gas inlet vessel 21 is communicated by means of valve V2 with a cell 11 that is maintained at an atmospheric pressure by being pre-filled with a sample gas or reference gas. After the communication, the gas pressure is measured by a pressure sensor 16 attached to the cell 11. Whether the exhalation collected in the breath bag is below a given amount or not can be accurately judged.