A memory device includes: a memory cell region; a peripheral circuit region; a memory cell array; a control logic circuit; and a row decoder. The row decoder is configured to activate string selection lines based on control of the control logic circuit. A program interval is formed between a first program operation and a second program operation. The control logic circuit includes a reprogram controller configured to control the row decoder so that a program interval differs in the memory cells connected to different string selection lines among the memory cells connected to a first wordline.