It is an object of the present invention to provide a method for killing tumor cells, having a few side effects. The present invention provides a method for killing tumor cells, comprising:(1) a step of allowing an immunotoxin formed by binding an antibody binding to ROBO1 or a fragment thereof to a cytotoxin to come into contact with tumor cells；(2) a step of allowing a photosensitizer that induces photochemical cytoplasmic internalization to come into contact with the tumor cells； and(3) a step of irradiating the tumor cells with a wave length that is effective for activating the sensitizer, so as to kill the cells.
