A flow path device comprises a plate-like measurement flow path device and a plate-like separation flow path device. The measurement flow path device includes a first flow path for measuring specific particles on a first fluid and connected to a third flow path and a second flow path for correction and passing a second fluid, not including the specific particles. The separation flow path device includes a fourth flow path for separating and selecting the specific particles from a sample and collecting a fluid. The separation flow path device is on the measurement flow path device's upper surface. The sample passes through a fifth flow path, the upper surface's opening, and flows into the fourth flow path from an opening in the separation flow path device's lower surface. The first fluid passes through the lower surface's opening, and flows into the first flow path from the upper surface's opening.