To uniformly dilute particulate matter contained in a sample gas, a diluter includes an inflow portion, a mixing portion, a discharge portion, a connection portion, and an introduction portion. The inflow portion allows a sample gas to flow in. The mixing portion has an inner diameter larger than that of the inflow portion, and mixes the sample gas with a dilution gas to generate a diluted sample gas. The discharge portion discharges the diluted sample gas. The connection portion has a first tapered part whose inner diameter increases from a side connected to the inflow portion toward a side connected to the mixing portion. The introduction portion introduces the dilution gas into an internal space from a position downstream of the connection between the first tapered part and the inflow portion.