Microfluidic devices provide a number of advantageous features for microscale biochemical systems for analysis and/or synthesis. A PDMS (polydimethylsiloxane) microchip, for instance, which has microchannels for electrophoretic separation can be easily fabricated through a molding process. Sealing of those channels does not need any elaborate bonding processes which are usually required for glass chips. We have been working on PDMS-based microfluidic devices for biomedical applications, where microreactors, microchips for capillary gel electrophoresis, and hydrophobic vent valves are successfully fabricated and operated. Fundamentals of PDMS-based microfluidic devices and their functions are described as well as the experimental results.