Flocculation state monitoring sensor

A flocculation state monitoring sensor comprising:

• a light emitting part configured to radiate light toward a measuring region in which a flocculation state is measured;
• a light receiving part configured to receive light scattered in a direction crossing an optical axis direction of the light emitting part;
• a nozzle for cleaning the light emitting part configured to eject a gas toward the light emitting part;
• a nozzle for cleaning the light receiving part configured to eject a gas toward the light receiving part;
• and
• a control means configured to control ejection of the gas from each of the nozzles,
• wherein the control means performs control to execute a cleaning period in which the gas is ejected toward the light emitting part from the nozzle for cleaning the light emitting part or the gas is ejected toward the light receiving part from the nozzle for cleaning the light receiving part, and
• a purging period in which a gas is discharged at a lower flow speed than in the cleaning period from the nozzle during a time between cleaning periods.