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; and determining a deterioration state of a storage area including the selected memory cells based on the generated quality estimation information.