An operating method of a memory system that includes a plural-level cell memory block capable of storing N-bit data in a single memory cell includes accessing a plural-level cell memory block in an N-bit cell mode, determining a degree of disturbance of the plural-level cell memory block, designating one or more memory cells in an erase state included in an open memory area of the plural-level cell memory block as an M-bit group, where M is an integer smaller than N, according to a result of the determination, and accessing the M-bit group in an M-bit cell mode.