Provided is a flocculation state monitoring sensor with which blockage of an ejecting part which ejects a gas towards a light emitting part and a light receiving part can be prevented, and which performs stable monitoring. A flocculation state monitoring sensor comprising: a light emitting part 3 which irradiates laser light towards a measuring region S which measures a flocculation state; and a light receiving part 4 which receives light scattered along a direction which intersects with a direction along an optical axis of said light emitting part 3, wherein the light emitting part 3 and the light receiving part 4 are cleaned by air being ejected from nozzles 21, 31 theretowards. A small amount of air is provided to the nozzles 21, 31 between cleaning periods to purge floe, etc.