By using a two-color dip-spectroscopy, we measured the fluorescence intensity from tryptophan in a water solution. The fluorescence intensity exponentially decreased as the laser intensity for the S-n <-- S-1 excitation increased. The phenomenon was analyzed by a rate-equation for a three-state model. The analysis shows that tryptophan with the Sn state has a radiationless relaxation process without any process through the S-1 state, and that the S-n --> S-1 internal conversion does not have a 100% yield. The branching ratio of the process is estimated to be 20%. The presented result clarifies in detail the real meaning of Kasha's rule. (C) 2003 Elsevier Science B.V. All rights reserved.