1. A program method of a multi-bit memory device, the program method comprising: programming first page data in a first region of a memory cell array; after the programming of the first page data, storing the first page data in a first buffer of a page buffer; after the storing of the first page data, programming second page data in the first region of the memory cell array; after the programming of the second page data, storing the second page data in a third buffer of the page buffer; after the storing of the second page data, programming third page data in the first region of the memory cell array; after programming of the third page data, transferring the second page data stored in the third buffer to a second buffer of the page buffer and then storing the third page data in the third buffer; and performing, based on the first to third page data stored in the page buffer, a first programming operation on a second region of the memory cell array, wherein the first region of the memory cell array includes a memory cell configured to store 1-bit data, and wherein the second region of the memory cell array includes a memory cell configured to store M-bit data, where M is a natural number of 3 or greater.