We present a versatile device structure that can mechanically magnify the sensitivity of stimuli-responsive structural-color hydrogel biochemical sensors. In our system, the structural-color hydrogel part and the stimuli-response hydrogel part are separately arrayed on a substrate with micro-step structures, causing the height difference between the two hydrogel parts. This height difference magnifies the shrinkage ratio to be larger in the color part than in the stimulus-responsive part, achieving the x5 - x10 high sensitivity of the hydrogel-based biochemical sensor. We applied ethanol-responsive and temperature-responsive hydrogels to our proposed device and confirmed the magnification of the sensitivity compared to previous sensors. We believe that our sensor device will be an effective approach to providing sensing with practical sensitivity for medical and other applications.