1. A nonvolatile memory device comprising: a memory cell including a cell transistor having a floating gate and a coupling capacitor connected to the floating gate; and a switching unit including a first terminal coupled to the coupling capacitor and a second terminal coupled to a bias terminal, and switching on or off between the coupling capacitor and the bias terminal based on a comparison result between a cell current flowing through the memory cell and a reference current during a program operation for programming the memory cell, and wherein the reference current has a current value corresponding to a target threshold voltage among various threshold voltages that a plurality of memory cells including the memory cell have after the program operation.