A memory device includes a memory cell array with memory strings including a first and second select transistor and memory cells between the first and second select transistors. Each memory string has a bit line connected thereto. A different word line is connected to each of the memory cells of a memory strings. A control circuit is configured to execute a first read operation in which data is read at the same time from memory cells connected to all the bit lines and a second read operation in which data is read from memory cells connected to a first subset of bit lines and a shield voltage is applied to a second subset of bit lines in the plurality of bit lines. The controller selects the first or second read operation for execution according to the number of read voltage levels required for determining data in the memory cells.