There is provided a coagulation treatment device configured to perform chemical injection control based on a coagulation state monitoring sensor, the coagulation treatment device being capable of accurately controlling the addition amount of a coagulant with the color and amount of particles taken into consideration. A coagulant is added to a coagulation tank 3 by a chemical injection device 4. A detection signal from a coagulation state monitoring sensor 10 provided at a coagulation device 3 is input to a controller 8 and the chemical injection device 4 is controlled. The coagulation state monitoring sensor 10 includes a light emitting unit configured to emit a laser beam toward coagulation treatment solution, and a light receiving unit having a light reception optical axis orthogonal to a light emission optical axis of the light emitting unit. A light reception signal intensity change range is corrected by using light reception signal intensity, and a floc formation state is determined based on a corrected value of the light reception signal intensity change range.