mechanism 162 the width w 2 of the gas supply path of the secondary flow between the flow path structuring members 113 and 114 is set by the moving mechanism 163 fig5 illustrates yet another example in this figure parts corresponding to fig4 are assigned identical codes and explanations of these parts are omitted in this example flow path structuring members 111 through 114 shaped similarly to the cross section of a nozzle such as illustrated in fig1 are attached to a base plate where a moving mechanism is connected to part of the base plate the widths of the flow paths can be adjusted with this structure as well fig6 illustrates further example in this example the mechanism is one wherein the flow path structuring members 113 and 114 are swiveled around support points 173 and 175 by cam mechanisms 171 and 172 that are rotationally angularly driven by a motor not shown to adjust set the flow path width between the flow path structuring members 113 and 114 in this way it is possible to adjust the width of the flow path of the primary flow and the width of the flow path of the secondary flow of the virtual impactor the flow rate of the primary flow can be adjusted by adjusting the width of the flow path of the primary flow moreover the sizes of the particles of the branch flow and the sizes of the particles subject to condensation can be adjusted through adjusting.