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We have observed the HN13C J=1-0 and DNC J=1-0 lines toward 18 massiveclumps, including infrared dark clouds (IRDCs) and high-mass protostellarobjects (HMPOs), by using the Nobeyama Radio Observatory 45 m telescope. Wehave found that the HN13C emission is stronger than the DNC emission toward allthe observed sources. The averaged DNC/HNC ratio is indeed lower toward theobserved high-mass sources (0.009\pm0.005) than toward the low-mass starlessand star-forming cores (0.06). The kinetic temperature derived from the NH3 (J,K) = (1, 1) and (2, 2) line intensities is higher toward the observed high-masssources than toward the low-mass cores. However the DNC/HNC ratio of some IRDCsinvolving the Spitzer 24 {\mu}m sources is found to be lower than that ofHMPOs, although the kinetic temperature of the IRDCs is lower than that of theHMPOs. This implies that the DNC/HNC ratio does not depend only on the currentkinetic temperature. With the aid of chemical model simulations, we discuss howthe DNC/HNC ratio decreases after the birth of protostars. We suggest that theDNC/HNC ratio in star-forming cores depends on the physical conditions andhistory in their starless-core phase, such as its duration time and the gaskinetic temperature.

DNC/HNC Ratio of Massive Clumps in Early Evolutionary Stages of High-Mass Star Formation