1. A decoding apparatus comprising: a hard-decision unit performing hard-decision on a soft-input of a code; a reliability determination unit generating a reliability estimation value of a result of the hard-decision, wherein the reliability estimation value depends on whether a packet error exists or does not exist in the hard-decision result, wherein the hard-decision unit performs hard-decision depending on the reliability estimation value, wherein the reliability determination unit generates the reliability estimation value, which changes depending on the hard-decision result, when the packet error does not exist in the hard-decision result, and the reliability determination unit generates a constant reliability estimation value when the packet error exists in the hard-decision result, wherein the reliability determination unit comprises a packet error detector for detecting the packet error, depending on the hard-decision result outputted from the hard-decision unit, wherein the reliability determination unit further comprises a reliability estimation value generator for generating the reliability estimation value of the hard-decision result, according to whether the packet error is detected, and wherein the reliability estimation value generator further comprises a multiplexer for selecting either the hard-decision result value or a predetermined constant value, depending on whether the packet error is detected.