An injection control system of a flocculating agent comprises: a flocculating pool (15) into which the inflow liquid flocculating agent (11) is injected and which forms flocs of suspended matters in the liquid; flocculating agent injection means (12) for injecting the flocculating agent into the flocculating pool (15); floc image pickup means (18) for photographing a state of the flocs in the flocculating pool (15) and for converting luminance data of the flocs into an electric signal; image recognizing means (30) for recognizing the shape of floc by binarizing the image signal derived from the floc image pickup means (18) on the basis of a luminance level of each pixel; flocculation state deciding means (80) for calculating a characteristic amount of a diameter distribution of the flocs on the basis of the floc shapes recognized by the image recognizing means (30); and injection amount control means (100) for controlling an amount of the flocculating agent which is injected from the flocculating agent injection means (12) on the basis of the characteristic amount.