A light-trapping sheet of the present disclosure includes: a plurality of light-transmitting sheets each having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of each of the plurality of light-transmitting sheets at a first and second distance from the first and second principal surfaces, respectively. Each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer sandwiched therebetween. A refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting sheet; and a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers. The third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet.