The present invention is the overflow-preventing method capable of avoiding great amount of normalizing operation in each recursion step of turbo code MAP decoder and ensuring least quantizing bits independent of the code length as well as the overflow preventer based on the said method. Based on the highest bit of the state variable in the last recursion operation, the branch variable is performs addition/subtraction/no operation of some increment in the present recurrence operation. The adopting of the said method has operation precision dependent on only input quantizing level and independent on the code length. The relevant operation includes checking the highest two bits of the state variables and increment or decrement of some fixed number only, and the very little operation amount results in little effect on the decoding speed.