Quantum dot refers to a particle of a few nanometer size, made of inorganic semiconductor. When irradiated with ultraviolet rays, it gives strong fluorescent emission. It has been regarded as a means for imaging genes and proteins in cells, as well as cancer cells within the body. The SMBL-AIST has developed technique to prepare the quantum dot in a simple way, and that for combining antibody or lectin identifying cancer cells with quantum dot. In this way, technology for distinguishing cancer cells from normal ones has been successfully developed. Moreover, it has been found that cancer cells selectively ingested quantum dots are readily killed by the irradiation with ultraviolet rays. This is the first achievement in the world to clearly show that quantum dots are applicable not only to molecular imaging but also to photodynamic cancer therapy.