Laser manipulation techniques were applied to the control of molecular assembling in D2O. An amphiphilic random copolymer, which has hydrophilic and hydrophobic segments in a single chain and forms a monopolymer micelle through self-organization, is demonstrated to receive photon pressure of a focused near-infrared laser beam. Upon prolonged irradiation with the beam, a single microparticle is formed at a focal point, and it dissolves very quickly after switching off the laser beam. The behavior indicates that electrostatic repulsion between micelles is overcome by photon pressure. The assembling processes were examined as a function of irradiation time and polymer concentration and are discussed.