We conducted a study to determine the threshold intensity of twilight illumination for photoperiodism in rice (Oryza sativa L.) based on solar altitude in an open-field, day-length-extension experiment and utilization of a crop model. Light supplementation was provided by incandescent lamps and controlled based on solar altitude using four japonica rice cultivars: Hitomebore, Koshihikari, Nipponbare, and Nikomaru. First, pot experiments were conducted individually with supplementary light treatment in the morning and in the evening. By comparing the difference in heading time between rice with and without supplementary light, we determined the threshold intensity of twilight illumination. The threshold was between a solar altitude of 0 degrees and -2 degrees in the morning and evening civil twilight. Second, a field experiment was conducted with light supplementation in the morning and evening civil twilight. Based on the results, we concluded that the threshold intensity of illumination would be around a solar altitude between -1 degrees and -2 degrees in the morning and the evening civil twilight.