Fine particle measurement device

A fine particle measurement device comprising:

• a 4f optical system in an optical path that causes a beam spot of a laser output from a light source to form an image with respect to fine particles,
• wherein the 4f optical system is a relay lens system that is disposed between a first imaging lens and a second imaging lens that configure an imaging lens system of the beam spot,
• wherein a first relay lens that configures the relay lens system is disposed in a position in which the distance between the first relay lens and the first imaging lens is equal to the focal point distance of the first relay lens,
• wherein a second relay lens that configures the relay lens system is disposed in a position in which the distance between the second relay lens and the first relay lens is equal to the sum of the focal point distance of the first relay lens and the focal point distance of the second relay lens, and the distance between the second relay lens and the second imaging lens is equal to the focal point distance of the second relay lens,
• wherein an optical filter is disposed in one or more positions selected from between the first imaging lens and the first relay lens, between the first relay lens and the second relay lens, and between the second relay lens and the second imaging lens, and
• wherein the optical filter is a mirror that reflects fluorescent light or scattered light that is generated from the fine particles as a result of irradiation with the laser, or a splitter that demultiplexes the laser.