It has been believed that a molecule absorbs one photon on its photoexcitation. This is true for most cases. However, under strong irradiation of light like a pulsed laser beam, two photons are absorbed at the same time by a single molecules - this is two-photon absorption (TPA). Generally organic dyes show very weak TPA. We developed dyes exhibiting very strong TPA. The dyes consist of diacethylene derivatives and is stable compared to other TPA dyes. The dyes also show drastic increase of the TPA sensitivity for visible ray because of the mechanism called resonance enhancement (near double resonance). These supersensitive TPA dyes have high potential for the application such as high-density optical storage, 3D-microfabricaion, and photodynamic therapy.