1. A method of estimating a deterioration state of a memory device, comprising: reading data from selected memory cells connected to a selected wordline of a memory cell array by applying to the selected wordline a plurality of distinct read voltages having values corresponding to at least one valley of threshold voltage distributions of the selected memory cells; generating quality estimation information indicating states of the threshold voltage distributions using the data read from the selected memory cells by determining, based on the data read from the selected memory cells by applying each of the plurality of read voltages, a number of memory cells in each of multiple threshold voltage sections defined by the plurality of read voltages; and determining, based on the determined number of memory cells according to the threshold voltage sections, first type quality estimation information indicating read voltage displacement and a threshold voltage distribution width level; and determining a deterioration state of a storage area including the selected memory cells based on the generated quality estimation information.