Semiconductor device and operating method of semiconductor device

A method of operating a semiconductor device including memory blocks sharing a source line, each of the memory blocks including memory strings each including a first source select transistor, a second source select transistor and memory cells, the first source select transistor being located closer to the source line than the second source select transistor, the method comprising:

• applying a first voltage to a first source select line coupled to first source select transistors of memory strings included in an unselected memory block;
• applying a precharge voltage to the source line when the first voltage is applied to the first source select line;
• floating the first source select line after the first voltage is applied thereto;
• applying a second voltage having a lower voltage level than the first voltage to a second source select line coupled to second source select transistors of the memory strings included in the unselected memory block;
• and
• applying a program voltage to a word line coupled to selected memory cells of memory strings included in a selected memory block, among the memory blocks.