  We report the first detection of HC$_{5}$$^{15}$N with the $J=9-8$ rotational
line from the cyanopolyyne peak in Taurus Molecular Cloud-1 (TMC-1 CP) using
the 45-m radio telescope of the Nobeyama Radio Observatory. The column density
of HC$_{5}$$^{15}$N is derived to be (1.9 +- 0.5)*$10^{11}$ cm$^{-2}$ (1
sigma). We apply the double isotope method to derive the $^{14}$N/$^{15}$N
ratios of HC$_{5}$N and HC$_{3}$N in TMC-1 CP. The $^{14}$N/$^{15}$N ratios are
calculated to be 344 +- 53 and 257 +- 54 for HC$_{5}$N and HC$_{3}$N,
respectively. The $^{14}$N/$^{15}$N ratio of HC$_{5}$N is lower than the
elemental ratio in the local interstellar medium (~440) and slightly higher
than that of HC$_{3}$N in TMC-1 CP. Since HC$_{3}$N is formed via the
neutral-neutral reaction between C$_{2}$H$_{2}$ and CN, the slightly higher
$^{14}$N/$^{15}$N ratio of HC$_{5}$N may support our previous suggestions that
the main formation mechanism of HC$_{5}$N is the ion-molecule reactions between
hydrocarbon ions (C$_{5}$H$_{n}^{+}$) and nitrogen atoms.
