Space-selective phase separation was induced inside Na(2)O-SiO(2) glasses by changing the glass composition locally from a miscible composition to an immiscible one via femtosecond-laser irradiation at a high repetition rate. The formation of a nanoscale co-continuous structure due to phase separation was confirmed by scanning electron microscopy after heat treatment, polishing until the laser-modified area was exposed, and subsequent etching. The configuration of the structure could be controlled by changing the heat-treatment time.