We have investigated singlet oxygen generation and photocytotoxicity against tumor cells by two-photon excitation using a two-photon absorbing photosensitizer consisting of a butadiyne-bridged porphyrin dimer terminated with two water-soluble porphyrin monomers. The compound exhibited high efficiency for singlet oxygen generation under not only one-photon but also two-photon irradiating conditions. The successful destruction of Hela cancer cells via two-photon absorption (2PA) is demonstrated.