We attained the novel micelle formation induced by photolysis of a diblock copolymer. A poly(tert-butoxystyrene)-block-polystyrene diblock copolymer (PBSt-b-PSt) with the molecular weight of Mn(PBSt-b-PSt) = 15000-b-97000 showed no self-assembly in dichloromethane and existed as isolated copolymers with the hydrodynamic diameter of 16.6 nm. Dynamic light scattering demonstrated that the copolymer produced micelles with a 63.0 nm hydrodynamic diameter when the copolymer solution in dichloromethane was irradiated with a high-pressure mercury lamp at room temperature in the presence of bis(alkylphenyl) iodonium hexafluorophosphate, a photoacid generator. The 1H NMR analysis revealed that the micellization resulted from the photolysis of the PBSt blocks into insoluble poly(vinyl phenol) blocks based on the fact that the signal intensity of the tert-butyl protons decreased over time during the irradiation. It was found that the micellization rapidly proceeded as the degree of the photolysis reached over 50% and was completed at 90%. The micellization was dependent on the molecular weight of the copolymer and on the type of photoacid generator.