A wastewater treatment method comprising: a mixing step of mixing ballast wastewater with scrubber wastewater to make a suspended solid concentration or turbidity of the obtained wastewater mixture constant, the scrubber water produced by bringing exhaust gas and scrubber washing water into contact with each other in a scrubber; a treatment step of adding a magnetic powder to the wastewater mixture; and a separation step of magnetically separating magnetic flocs obtained in the treatment step; wherein the mixing step includes: a measurement step of measuring the suspended solid concentration of the scrubber wastewater or the suspended solid concentration of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step; or wherein the mixing step includes: a measurement step of measuring the turbidity of the scrubber wastewater or the turbidity of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step. a mixing step of mixing ballast wastewater with scrubber wastewater to make a suspended solid concentration or turbidity of the obtained wastewater mixture constant, the scrubber water produced by bringing exhaust gas and scrubber washing water into contact with each other in a scrubber; a treatment step of adding a magnetic powder to the wastewater mixture; and a separation step of magnetically separating magnetic flocs obtained in the treatment step; wherein the mixing step includes: a measurement step of measuring the suspended solid concentration of the scrubber wastewater or the suspended solid concentration of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step; or wherein the mixing step includes: a measurement step of measuring the turbidity of the scrubber wastewater or the turbidity of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step. a measurement step of measuring the suspended solid concentration of the scrubber wastewater or the suspended solid concentration of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step; or wherein the mixing step includes: a measurement step of measuring the turbidity of the scrubber wastewater or the turbidity of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step. a measurement step of measuring the turbidity of the scrubber wastewater or the turbidity of the wastewater mixture; and a control step of controlling a mixing ratio of the ballast wastewater and the scrubber wastewater depending on a measurement result in the measurement step.