Abstract: There is provided a method and apparatus of detecting minute impurities in the entire body of a fluid. Part of the passage through which the fluid flows is constituted by a transparent member. Light is emitted in a direction substantially parallel to the longitudinal axis of this part of the fluid passage so as to envelop the same. Light scattered by impurities in the fluid is observed from a direction substantially perpendicular to the direction of the passage to detect any minute impurities in the fluid. The method and apparatus is well-suited for use in systems which extrude molten resin in the formation of plastic products used in an electrical environment in order to detect impurities in the resin which could adversely affect the performance of the products.