Detecting the chirality of material is very important in material science, chemistry, pharmacology, and biomaterial science because it is useful for identifying and discarding unintended side effects. Here, directly visualized is the optical activity of chiral samples using an orientation-controlled helical nanofilament (HNF, B4) liquid crystal (LC) phase made of achiral bent-shaped molecules. The vertical orientation of the HNFs is induced by shining unpolarized UV light. Right- or left-handed domains of the HNFs are formed because of the lack of molecular chirality, each single-handed domain is large enough to be seen with the naked eye, up to approximate to several mm(2). The periodic arrays of aligned HNFs reflect a specific color, here green, due to the Bragg reflection. Such a reflector enables an easy detection of optical activity of a sample placed on it. The device is tested with naturally chiral substances, like fructose and glucose, which exhibit opposite sense of optical activity, as well as with structurally chiral nematic LC phase and reveals high sensitivity of a detection.