An operation method of a controller, comprising: selecting a target super block, on which garbage collection (GC) is to be performed, among a plurality of super blocks which are completely programmed, based on a first valid page count of each of the super blocks when a determination to perform GC is made; selecting a first target block among a plurality of memory blocks in the target super block based on a second valid-page decrease amount of each of the memory blocks; and performing a first copy operation on valid pages in the first target block.