A test system with a fuzzy logic controller is proposed to assure stable outgoing quality as well as to raise throughput. The test system controls the number of items under test in accordance with fuzzy information as well as statistical information about incoming quality and outgoing quality. First, an algorithm, minimum-minimum-the center of gravity-weighted mean method, is studied with both fuzzy reasoning rules and membership functions which are used for the control. Second, characteristics of the test system are verified and examined with computer simulations so that the fuzzy logic control rules are determined to realize sufficient sensitivity to process changes. Third, the control rules are installed in the test management processor which commands test equipment for testing very large scale integrated circuits, with programming language C. The authors have obtained satisfactory results through a trial run using a series of lots of 16 bit micro controller units in an IC manufacturing factory. Finally, they study the stability condition of the fuzzy test system.