A row hammer preventing circuitry including: a first table storing a count value representing a hit count and an address bit of multiple entries, each entry corresponding to access-requested target rows; a second table including safe bits and a safe bit counter; and a row hammer preventing logic to identify masking entries, on which a masking comparison is to be performed, among the entries on the basis of the safe bit counter, to determine a hit or miss on the basis of whether other bits except an MSB among address bits of an access-requested target row match other bits except an MSB among address bits of the masking entries, and to generate a control signal indicating an additional refresh on rows adjacent to rows corresponding to a masking entry whose hit count is greater than a threshold value.