The present invention provides a novel sensor for detecting a target. The nucleic acid sensor of the present invention includes a nucleic acid element that includes a catalyst nucleic acid molecule (D) that exerts a catalytic function and a binding nucleic acid molecule (A) that binds to a target. The nucleic acid element is a double-stranded nucleic acid element including a first strand and a second strand. The first strand (ss1) includes the binding nucleic acid molecule (A), a loop-forming sequence (L1), and the catalyst nucleic acid molecule (D) linked in this order. The second strand (ss2) includes a stem-forming sequence (SA), a loop-forming sequence (L2), and a stem-forming sequence (SD) linked in this order. In this nucleic acid element, in the absence of a target, the catalytic function of the catalyst nucleic acid molecule (D) is inhibited by stem formation in each of the stem-forming sequences (SA) and (SD), and in the presence of ATP/target, the stem formation is released by a binding of the binding nucleic acid molecule (A) with the target, and the catalytic function is exerted.