A mask ROM having a defect repairing function stores address signals corresponding to a defective memory cell and then, selectively activates either a redundancy row decoder or a row decoder according to whether the address signals stored are identical to address signals supplied externally. The mask ROM includes first and second memory cell arrays formed by grouping in a word line direction a plurality of read only memory cells arranged in rows and columns; first and second row decoders for combining row address signals supplied externally so as to selectively drive the word lines of the first and second memory cell arrays; and a row decoder selector for storing therein address signals according to a row block including a defective memory cell, of the first memory cell array so as to inactivate the first row decoder and activate the second row decoder when the external row address signals are equal to the address signals stored in the row decoder selector.