A data storage device includes a nonvolatile memory device; and a control unit suitable for controlling a program operation for memory cells of a page of the nonvolatile memory device, and processing a program fail in the case where the program operation fails, wherein the control unit adjusts a read voltage for discriminating an erase state and a program state having a threshold voltage most adjacent to the erase state, reads out data by applying the adjusted read voltage to the memory cells of the page, and performs an error handling operation to data stored in the memory cells of the page according to a result of comparing a reference value and a number of flipped bits of the data read out by applying the varied read voltage.