In this study, we propose a novel micro-/nanofluidic device that can generate a chemical concentration gradient using a parallel nanochannel as gradient generator. This device is easy to fabricate, showing high reproducibility. Its main feature is the multiple-nanochannel-based gradient generator, which permits the diffusion of small molecules and tunably generates concentration gradients. The nanopattern for the nanochannels can be rapidly and easily fabricated by wrinkling a diamond-like carbon thin film which is deposited on a polydimethylsiloxane substrate; the generation of the concentration gradient can be adjusted by controlling the dimensions of the nanochannels. The developed gradient generator is embedded into a microfluidic device to study chemotaxis in the nematode Caenorhabditis elegans, which has a highly developed chemosensory system and can detect a wide variety of chemical molecules. This device shows good performance for rapid analysis of C. elegans chemotaxis under sodium chloride stimuli.