A method of operating a semiconductor device includes storing a supplying condition of a read voltage inputted from an external source into an internal register to perform a read operation of memory cells, performing the read operation repetitively with changing levels of the read voltage according to the supplying condition of the read voltage in the event that the number of error bits in a data read from the memory cells exceeds an allowable range, and storing an iteration number of the read operation in the internal register in case the number of the error bits falls within the allowable range.