A hammer refresh row address detector includes a control logic unit that receives a row address applied along with an active command, to increase a hit count stored in a corresponding entry when the row address is present in candidate aggressor row addresses stored in n entries. The control logic determines a candidate aggressor row address stored in an entry in which the hit count equals a threshold value to be a target aggressor row address. The control logic generates a victim row address adjacent to the target aggressor row address as a hammer refresh row address to accompany a hammer refresh command. The control logic increases the miss count value when the row address is not present in the candidate aggressor row addresses stored in the n entries and no hit count within the n entries is identical to the miss count value.