A unique pillar structure of methylsiloxane gel in microfabricated grooves was developed through the competition of phase separation and wetting. In the course of the phase separation in the methylsiloxane sol-gel system, the siloxane phase completely wetted the surface of the grooves while forming material transfer channels, which were arrested as a permanent structure by the sol-gel transition. The phase-separated structure shows an isotropic weblike bicontinuous structure in the grooves wider than the critical value. However, in the narrower grooves, only the pillar structure remained, showing a web-to-pillar transition. The critical value is nearly equal to the characteristic length of the bulk.