To provide a composition with selective action which is capable of selectively acting on cancer cells without a fear of side effect, and a laser irradiation apparatus for treatment which effectively kills cancer cells and illness cells by a visible laser beam.

A ligand or an antibody 14 which reacts with a membrane protein which is expressed specifically or redundantly in the cancer cell, is incorporated into the cell and transported to a cell nucleus to be cross-linked to semiconductor crystals 11 having a diameter between 4 nm and 10 nm. The semiconductor crystal 11 in which the ligand or the antibody 14 combines with the membrane protein of the cancer cell is accumulated near the nucleus of the cancer cell. The semiconductor crystal accumulated near the nucleus of the cancer cell emits an active oxygen by visible light irradiation and selectively kills the cancer cell.