Disclosed herein is a detecting nucleic acid strand including: a first nucleic acid strand having a first base sequence region capable of functioning as an aptamer, and a second nucleic acid strand having a second base sequence region, which is complementary to the first base sequence region and forms a complementary strand with the first base sequence region, wherein the detecting nucleic acid strand is designed such that, when a predetermined substance interacts with the first base sequence region, the complementary strand of the first and second base sequence regions dissociates into single strands.