The present technology provides a method of operating a semiconductor memory device detecting a threshold voltage distribution for memory cells included in a page selected from among a plurality of memory cells. The method of operating the semiconductor memory device includes selecting a target state in which the threshold voltage distribution is to be detected, determining a plurality of read voltages for dividing a voltage range in which a threshold voltage of the selected target state is distributed, and performing a plurality of sensing operations using the plurality of read voltages on the selected page. Masking to the target state is applied in each of the plurality of sensing operations.