Pixel electrodes on an active-matrix substrate do not overlap signal lines and scanning lines, and satisfy a relation X1>Y1, where X1 is the gap between the pixel electrodes and the signal lines, and Y1 is the gap between the pixel electrodes and the scanning lines. As a result, a large-area and high-resolution electromagnetic wave detector is realized with an improved S/N ratio, even with an active-matrix array providing a large area for the charge collecting electrodes.