To obtain an apparatus for measuring a distribution of contaminated particles in a working oil which measures a particle size with the utilization of light transmission holes of different sizes of a plurality of measurement units and widens a measurable range of the particle size.

A plurality of measurement units 12 are set at a working oil path 11 of a press machine. Each measurement unit 12 is constituted of a measurement cell 21 having a measurement path 22 communicating with the working oil path 11, a pair of emission and reception rod-shaped light-transmitting bodies 24, 25 penetrating a peripheral wall of the measurement path 22 from inside to outside and arranged to confront each other via a measurement gap between inner end faces, a light-emitting device 26 disposed at an outer end of the emission light-transmitting body 24, a photodetector 27 disposed at an outer end of the reception light-transmitting body 25 and a photomask 28 interposed between the outer end of the reception light-transmitting body 25 and the photodetector 27 and having a light transmission hole. Sizes of the light transmission holes of photomasks 28 of all measurement units 12 are changed stepwise so that the light transmission hole of the photomask 28 of each measurement unit 12 is adaptable in size to a specific range only among an entire range of measurement particle sizes.