To solve a problem that a conventional mouthpiece with a filter has a complicated constitution, resulting in a high cost, there is provided a mouthpiece, including: a tubular mouthpiece main body that forms a flow path for flowing a breathing gas a filter placed on the flow path a tubular insertion member inserted into the mouthpiece main body, wherein the filter has a first filter portion corresponding to a cross-sectional shape of the flow path and a second filter portion folded-back from an outer circumferential edge of the first filter portion, and the second filter portion is sandwiched between an inner circumferential surface of the mouthpiece main body and an outer circumferential surface of the insertion member.
