Ionic conductivity of NASICON-type Na(1+x)M(x)Zr(2-x)P(3)O(12) was investigated associated with the behavior of the crystal parameters. The conductivity increased showing the maximum at x similar to 1.0 by about three orders of magnitude. When the conductivity was separated into the two essential factors, mobile ion concentration and hopping rate, using a frequency dispersion analysis, the hopping rate was dominantly enhanced and in good agreement with the behaviour of c-parameter change with x. This suggests that the bottleneck size reflecting the c-axis length dominates the ionic conduction of the skeleton structure.