A carboxy tetraphenylporphyrin derivative formed aggregate by decreasing its pH, and suppressed its fluorescence and sensitization of singlet oxygen. This indicates that ON/OFF switching of the photosensitization can be achieved by aggregation and disaggregation of photosensitizers. To induce disaggregation, ultrasound was applied to the solution of the aggregate of photosensitizers, and disaggregation was confirmed by studying the fluorescence of the photosensitizer and phosphorescence of singlet oxygen sensitized. Thus this system works as a sono-activatable photosensitizer for photodynamic therapy.