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We observed the J = 9-8 and 10-9 rotational lines of three ¹³C isotopologues of HC₃N in L1527 and G28.28-0.36, with the 45 m radio telescope of the Nobeyama Radio Observatory, in order to constrain the main formation mechanisms of HC₃N in each source. The abundance ratios of the three ¹³C isotopologues of HC₃N are found to be 0.9 (±0.2) : 1.00 : 1.29 (±0.19) (1σ), and 1.0 (±0.2) : 1.00 : 1.47 (±0.17) (1σ), for [H¹³CCCN : HC¹³CCN : HCC¹³CN] in L1527 and G28.28-0.36, respectively. We recognize, from a similar ¹³C isotopic fractionation pattern, that the abundances of H¹³CCCN and HC¹³CCN are comparable, and HCC¹³CN is more abundant than the others. Based on the results, we discuss the main formation pathway of HC₃N. The ¹³C isotopic fractionation pattern derived from our observations can be explained by the neutral-neutral reaction between C₂H₂ and CN in both the low-mass (L1527) and high-mass (G28.28-0.36) star-forming regions.