We present the first determination of the abundance ratios of 13 C substitutions of cyanoacetylene (HC 3 N),[H 13 CCCN]:[HC 13 CCN]:[HCC 13 CN], in Titan's atmosphere measured using millimeter-wave spectra obtained by the Atacama Large Millimeter/submillimeter Array. To compare the line intensities precisely, data sets that include multiple molecular lines were extracted to suppress effects of Titan's environmental conditions and observation settings. The [HC 13 CCN]:[HCC 13 CN] and [H 13 CCCN]:[HCC 13 CN] ratios were obtained from 12 and 1 selected data sets, respectively. As a result, nearly the uniform [H 13 CCCN]:[HC 13 CCN]:[HCC 13 CN] abundance ratios of 1.17 (±0.20): 1.09 (±0.25): 1 (1σ) were derived, whereas previously reported ratios for the interstellar medium (ISM) have shown large anomalies that may be caused by 13 C concentrations in precursors. The result obtained here suggests that 13 C concentration processes suggested in the ISM studies do not work effectively on precursors of HC 3 N and HC 3 N themselves owing to Titan's high atmospheric temperature and/or depletion of both 13 C and 13 C+.