Aptamers are single-strand oligonucleotides that are generated by the systemic evolution of ligands by exponential enrichment (SELEX) technique and that can bind to target molecules specifically. However, only a few aptamers have been developed to date against tumor markers. To utilize aptamers for tumor diagnosis, a variety of aptamers are required. Here, a single-stranded DNA aptamer specific for pro-gastrin-releasing peptide (proGRP), a marker for small cell lung cancer, was selected using SELEX. After selection, identical sequences were found in the DNA library. This sequence was selected and its binding affinity to proGRP was evaluated using surface plasmon resonance.