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We observed the J=5-4 rotational lines of the normal species and three ¹³C isotopologues of HC₃N 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 HC₃N in each core. The abundance ratios of the three ¹³C isotopologues in L1521B are derived to be [H¹³CCCN]:[HC¹³CCN]:[HCC¹³CN] = 0.98 (±0.14):1.00:1.52 (±0.16) (1σ ). 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 HC₃N is the neutral-neutral reaction between C₂H₂ and CN. On the other hand, their abundance ratios in L134N are found to be [H¹³CCCN]:[HC¹³CCN]:[HCC¹³CN] = 1.5 (±0.2):1.0:2.1 (±0.4) (1σ ), 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 HC₃N in L134N. We find out that the main formation pathways of the same molecule are not common even in similar physical conditions. We discuss the possible factors that could make a difference in the fractionation pattern between L134N and L1521B/TMC-1.