Abstract: Provided is a microplastic discharge amount assessment method with which it is possible to objectively quantitatively assess the discharged amount of microplastic discharged from a textile product containing a synthetic resin. The microplastic discharge amount assessment method is characterized by including: a cleaning step for retrieving a test piece of a specified size or mass from a textile product containing a synthetic resin, and cleaning said test piece; a filtering step for using a filter to filter all wastewater discharged in the cleaning step; a measurement step for measuring the mass of textile waste trapped by the filter; and an assessment step for assessing the discharged amount of microplastic discharged from the textile product, on the basis of the measurement result from the measurement step.