A memory system including: a memory device having an open block and a closed memory block; a page counting unit counting the number of program pages in the open block whenever a data is programmed in the open block, and counting the number of valid pages of the closed memory block; a valid page decrease amount counting unit calculating a total sum of valid pages decreased in the closed memory block before and after a map update operation; and a garbage collecting unit performing a garbage collection operation onto a victim block when the number of free blocks included in the memory device is less than a first threshold value and greater than a second threshold value, and a ratio of the number of the program pages in the open block to the total sum of the valid pages decreased is greater than or equal to a fourth threshold value.