A measuring device can be used under high pressure and can measure impurity particles contained in a hydraulic oil with high accuracy. A flow path hole opening on two facing surfaces of a housing has flat side surfaces. A cavity opens on the side surface, and a cavity opens on the side surface. Light emitted from a light irradiating section irradiates a hydraulic oil flowing in the flow path hole, via a cell disposed in the cavity in a direction substantially orthogonal to a center axis. Light passing through the hydraulic oil is received by a light receiving section via a cell disposed in the cavity opening on the side surface.