An object of the invention is to provide a wastewater treatment system using a superconducting magnetic separator that can wash the magnetic filter efficiently. The system comprises in a bore of a superconducting magnet a removably built-up multiunit magnetic filter consisting of a plurality of single-unit magnetic filters. The multiunit magnetic filter has a total longitudinal length at least equal or greater than that of the superconducting magnet. During excitation of the superconducting magnet, a single-unit magnetic filter is removed from the upstream side (sewage side) of the multiunit magnetic filter by pushing in another single-unit magnetic filter from the downstream side (clean water side), and washed and returned to the downstream side again.