Semiconductor memory device controlling operation timing of the sense circuit

A method of reading data from a semiconductor memory device including a plurality of memory strings including memory cells connected in series and stacked above a semiconductor substrate, the method comprising:

• selecting a first memory string from the memory strings and allowing a cell current to flow in a bit line electrically coupled to the first memory string;
• generating a reference current based on currents flowing through source lines electrically coupled to unselected second memory strings;
• sensing the reference current;
• and
• sensing the cell current with the timing based on the result of sensing the reference current,
• wherein the sensing the cell current is performed on a plurality of columns simultaneously,
• second sensing is performed on a column in which a memory cell has been determined to be off after the cell current has been sensed, and
• a magnitude of the reference current or a number of reference sense circuits which sense the reference current are variable according to a number of memory cells to be sensed.