A check node update processor of the low density parity check (LDPC) code decoder includes: an approximate first minimum (AFM) condition check unit which checks whether a predetermined specific condition is satisfied, and a check node determining unit which sets an approximate minimum value as a size of an entire check node output when it is determined that the specific condition is satisfied as a checking result in the AFM condition check unit and calculates a first minimum value as a true minimum value and sets a second minimum value as an approximate minimum value when it is determined that the specific condition is not satisfied to determine a size of the check node output.