1. A most likely code word determiner provided in a decoder of a digital data transmission system using error-correcting codes for combating noisy environments, comprising: a plurality of comparators which respectively receive a plurality of metrics each indicating closeness of a corresponding code word to a signal transmitted to the decoder, said plurality of comparators respectively comparing said plurality of metrics with a threshold and generating a plurality of comparison results; a counter coupled to receive said plurality of comparison results, counting a number of comparison results each of which indicates that the corresponding metric exceeds said threshold, and generating a count result indicating that said number of comparison results is zero or one or more than one; a controller coupled to receive said count result and issuing an acknowledge signal indicating that said count result is zero or one or more than one, said controller changing said threshold if said count result is zero or more than one and applying the changed threshold to said plurality of comparators; and a selector coupled to receive said plurality of comparison results and a plurality of code words previously prepared in the decoder, said selector selecting a code word which is defined as said most likely code word if said acknowledge signal indicates that said count result is one.