A nonpoint source pollutant reducing apparatus capable of automatically controlling the flow rate of first flush without using power is provided to prevent the deterioration of treatment efficiency or the generation of a backflow phenomenon by preventing problems due to overload of a first flush treatment tank and preventing floating adulterations from flowing into the treatment tank. A nonpoint source pollutant reducing apparatus capable of automatically controlling the flow rate of first flush without using power comprises: a flow equalization tank(100) for pre-treating the first flush by guiding first flush through an inlet(110) communicating with a canal; a first flush treatment tank(200) into which the first flush of the flow equalization tank flows via a communication path(210), and in which a filter screen(220) is installed to remove nonpoint source pollutants contained in the first flush; and a flow equalization unit(300) installed on the communication path to block first flush of a planned treatment quantity or more flowing into the first flush treatment tank or allowing the first flush to flow into the first flush treatment tank constantly, wherein a filter screen is formed in the center of the first flush treatment tank, the first flush flown in from the communication path flows from an outer part of the filter screen to an inner part thereof, and an outlet side of the filter screen has perforated holes larger than those of an inlet side of the filter screen. The flow equalization unit is comprised of a floating type automatic cutout valve(310).
