A sensor includes a gel for a sensor and an electrode. The gel for a sensor includes a stimulus-responsive gel, first particles which are contained in the stimulus-responsive gel and are constituted by a material containing an electrically conductive substance, and second particles which are contained in the stimulus-responsive gel and contribute to structural coloration. It is preferred that when the electrical resistivity of the electrically conductive substance is represented by R1 (Ω·m) and the electrical resistivity of the constituent material of the second particles is represented by R2 (Ω·m), the following relationship is satisfied: 1×1012≦R2/R1≦1×1023. It is preferred that when the refractive index of the constituent material of the first particles is represented by n1 and the refractive index of the constituent material of the second particles is represented by n2, the following relationship is satisfied: −0.8≦n2−n1≦1.5.
