A memory system includes: a memory device; a valid page counter for counting the number of valid pages of each closed block in the memory device before and after a map update operation; a maximum valid page decrease amount detector for detecting a maximum valid page decrease amount by calculating a valid page decrease amount for each closed memory block based on the number of the valid pages for the corresponding closed memory block, so as to calculate a total sum of valid page decrease amounts; and a garbage collector for performing a garbage collection operation on a victim block in the memory device, when the number of free blocks in the memory device is less than a first threshold value and greater than a second threshold value and a ratio of the maximal valid page decrease amount to the total sum of the valid page decrease amounts is a third threshold value or less.