An inertial filter to be disposed in the air flow path of a stream of air containing nanoparticles includes plural particle classifying sheets having plural mesh-like holes are disposed in the air flow path. The plural particle classifying sheets are stacked on each other in a manner that a part of at least one of the particle classifying sheets overlaps with the other particle classifying sheet. The inertial filter can adjust a classification diameter by adjusting a shadow area ratio on a projection view obtained by virtually projecting the plural particle classifying sheets from a downstream side to an upstream side of the air flow path.