A method for operating the 3D NAND device includes providing first and second dies and initial read levels for the first and second dies, changing the initial read level for the first die to a first read level based on a first offset that is calculated in consideration of elapsed time from a time point when a program for the first die is completed, changing the initial read level for the second die to a second read level based on a second offset that is calculated in consideration of elapsed time from a time point when a program for the second die is completed, and reading data stored in the first die using the first read level or reading data stored in the second die using the second read level.