  We observed the $J=5-4$ rotational lines of the normal species and three
$^{13}$C isotopologues of HC$_{3}$N at the 45 GHz band toward two low-mass
starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio
telescope in order to study the main formation pathways of HC$_{3}$N in each
core. The abundance ratios of the three $^{13}$C isotopologues in L1521B are
derived to be [H$^{13}$CCCN]:[HC$^{13}$CCN]:[HCC$^{13}$CN] = 0.98 (+- 0.14) :
1.00 : 1.52 (+- 0.16) (1 sigma). The fractionation pattern is consistent with
that at the cyanopolyyne peak in Taurus Molecular Cloud-1. This fractionation
pattern suggests that the main formation pathway of HC$_{3}$N is the
neutral-neutral reaction between C$_{2}$H$_{2}$ and CN. On the other hand,
their abundance ratios in L134N are found to be
[H$^{13}$CCCN]:[HC$^{13}$CCN]:[HCC$^{13}$CN]= 1.5 (+- 0.2) : 1.0 : 2.1 (+- 0.4)
(1 sigma), which are different from those in L1521B. From this fractionation
pattern, we propose that the reaction between HNC and CCH is a possible main
formation pathway of HC$_{3}$N in L134N. We find out that the main formation
pathways of the same molecule are not common even in the similar physical
conditions. We discuss the possible factors to make a difference in
fractionation pattern between L134N and L1521B/TMC-1.
