Gas is supplied from positive pressure (105 Pa or more) to an analyzer of high vacuum (10−2 Pa or less) precisely and stably, while keeping conditions constant and replicating the conditions, and performing switching to a desired gas within a short time. According to a gas introducing device and a method, a plurality of types of gases are synthesized in a mixing chamber, the synthesized gas is introduced and is decompressed by a decompression pump to a pressure ranging from 0.1 Pa to 0.1 MPa, and the decompressed gas is introduced to a gas analyzer through a switching operation using a gas switching valve.