We demonstrate, for the first time to the best of our knowledge, fabrication of three-dimensional microfluidic channels with arbitrary lengths and configurations inside glass by femtosecond laser direct writing. The main fabrication process includes two steps: (1) direct formation of hollow microchannels in a porous glass substrate immersed in water by femtosecond laser ablation and (2) postannealing of the glass substrate at $\sim 1150°\mathrm{C}$ by which the porous glass can be consolidated. We show that a square-wavelike channel with a total length of $\sim 1.4\mathrm{cm}$ and a diameter of $\sim 64\mathrm{\mu m}$ can be easily produced $\sim 250\mathrm{\mu m}$ beneath the glass surface.