If field-effect devices based on wide-band-gap semiconductors are ever to find wide application in real transparent electronics, their performance is required remain stable under visible light. In particular, a thin-film transistor (TFT) irresponsive to visible illumination has a practical advantage, if used for the switching of pixels in a liquid crystal display (LCD), because shadeless pixels make it possible to lower cost and power consumption. In this letter, we report such a TFT with a channel made of low-temperature-grown MgxZn1-xO film. This device with a polycrystalline Mg0.1Zn0.9O channel operated under illumination shows the same transfer characteristics as those in the dark when the wavelength is longer than 400 nm. The field-effect mobility was measured to be 0.8 cm2V-1s-1, comparable to that of amorphous Si-based TFTs used in commercial LCDs.