Concentrations of 4 trace elements, Mg, K, Ca and Mn, in each tissues of Japanese morning glory were analyzed during the flower development. To determine the element amount, neutron activation analysis with gamma-ray spectroscopy was performed. In this study, we focused on the movement of the trace elements especially with shortday treatment. Each element showed its specific distribution in the parts of a plant. The concentration of each element was changed rhythmically within a day. It was noted that, in the apical bud, concentrations of Mg, Ca and Mn were decreased from 5 h (2 h before light was on) to 7 h and increased again after 9 h.