As optical trapping by a focused laser beam is applied for nanoparticles, multiple particles are grasped in the focal spot and make an assembly. Gold nanoparticles confined in such a submicrometer optical cage show a characteristic extinction spectrum depending on laser power. The spectral change can be induced reversibly and repeatedly by tuning the laser power, demonstrating that the assembly of gold nanoparticles can be controlled by the gradient force. This is attributed to the soft confinement of nanoparticles dressed in electrostatic potential barriers.