A controller includes a processor suitable for reading target data based on a predetermined main read voltage, and on each of a plurality of candidate read voltages having different voltage values; a memory suitable for storing main coded data and candidate coded data which are obtained by reading the target data; an ECC suitable for decoding the main coded data to generate main decoded data, and decoding each of the candidate coded data to generate candidate decoded data; and a counter suitable for counting the number of error bits corresponding to the main decoded data, and counting each of numbers of error bits corresponding to each of the candidate decoded data; and a voltage setting circuit suitable for setting a candidate read voltage having a minimum number of error bits, among the candidate decoded data, and which is smaller than the number of error bits corresponding to the main decoded data, as the main read voltage.