In one embodiment of the present application, a stimulus-responsive gel is disclosed with optical characteristic molecule introduced therein that is capable of easily and securely converting a volume change responding to external stimulus to optical data being easy to handle in the construction of sensor system; an external stimulus measuring apparatus making use of the same; and a method of measuring external stimulus therewith. A molecule having fluorescent chromophore or a molecule or a molecule capable of absorbing visible light or ultraviolet light is introduced in a stimulus-responsive gel so that the content of the molecule having fluorescent chromophore or the molecule or a molecule capable of absorbing visible light or ultraviolet light falls within such a range that the concentration of the molecule having fluorescent chromophore or a molecule or the molecule capable of absorbing visible light or ultraviolet light in the stimulus-responsive gel with optical characteristic molecule introduced therein is substantially proportional to the fluorescence intensity or absorbance of the visible light or ultraviolet light of the stimulus-responsive gel with optical characteristic molecule introduced therein.