Na+-superionically conducting grass-ceramics were produced using Na5YSi4O12-type materials (Narpsio). Their conduction properties were experimentally found to depend upon the composition parameters x and y in Na3+3x-yY1-xPySi3-yO9; as a whole, the replacement of (SiO4)(4-) by (PO4)(3-) brought about the enhancement of conductivity, while the introduction of excess Na+ ions by substitution of 1Y(3+) with 3Na(+) did not give rise to a large decrease in conductivity. The latter result was explained on the basis of a structural consideration that the lattice sites of Y3+ accommodated some portion of Na+ ions.