We present observations of isotopic lines of dense gas tracers toward the nuclear region of nearby Seyfert 2 galaxy NGC 1068 with the IRAM 30 m telescope and the Atacama Pathfinder Experiment (APEX) 12 m telescope. We detected four isotopic lines ((HCN)-C-13 1-0, (HCO+)-C-13 1-0, (HNC)-C-13 1-0, and (HCO+)-O-18 1-0) at the 3 mm band with the IRAM 30m telescope and obtained upper limits of other lines. We calculated optical depths of dense gas tracers with the detected isotopic lines of HCN 1-0, HCO+ 1-0, and HNC 1-0. We find that the N-14/N-15 abundance ratio is greater than 420 if we adopt the upper limit of (HCN)-N-15(1-0) emission. Combining this with fluxes of 1-0 lines from IRAM 30 m observations and the upper limit of 3-2 lines from APEX 12 m observations, we also estimated the excitation condition of molecular gas in the nuclear region of NGC 1068, which is less dense than that in the extreme starburst regions of galaxies.
