A contactless Mask ROM is described, comprising a plurality of MOS-type memory cells. The memory cells include a plurality of first memory cells and a plurality of second memory cells. The first memory cells have a first channel conductivity so that they are depletion-mode MOS transistors, and the second memory cells have a second channel conductivity so that they are enhanced-mode MOS transistors. In the contactless Mask ROM, a memory cell shares two diffusions with two adjacent memory cells that are aligned with the memory cell along a first direction.