  Each of the nitriles present in the atmosphere of Titan can be expected to
exhibit different \ce{^{14}N/^{15}N} values depending on their production
processes, primarily because of the various \ce{N2} dissociation processes
induced by different sources such as ultraviolet radiation, magnetospheric
electrons, and galactic cosmic rays. For \ce{CH3CN}, one photochemical model
predicted a \ce{^{14}N/^{15}N} value as 120--130 in the lower stratosphere.
This is much higher than that for \ce{HCN} and \ce{HC3N}, $\sim$67--94. By
analyzing archival data obtained by the Atacama Large Millimeter/submillimeter
Array (ALMA), we successfully detected submillimeter rotational transitions of
\ce{CH3C^{15}N} ($J$ = 19--18) locate at the 338 GHz band in Titan's
atmospheric spectra. By comparing those observations with the simultaneously
observed \ce{CH3CN} ($J$ = 19--18) lines at the 349 GHz band, which probe from
160 to $\sim$400 km altitude, we then derived \ce{^{14}N/^{15}N} in \ce{CH3CN}
as 125$^{+145}_{-44}$. Although the range of the derived value shows
insufficient accuracy due to data quality limitations, the best-fit value
suggests that \ce{^{14}N/^{15}N} for \ce{CH3CN} is higher than values that have
been previously observed and theoretically predicted for \ce{HCN} and
\ce{HC3N}. This may be explained by the different \ce{N2} dissociation sources
according to the altitudes, as suggested by a recent photochemical model.
