The simplest form of microfluidic valve is proposed with detailed working mechanism. Conventional on-chip valves are requiring a multiple layers of microchip with complicated fluidic pattern and/or complicated external control system. In contrast, our developed valve can be fabricated in a bilayer microchip with a simple pattern, two channels and a wall separating them. Our valve fabrication was based on a plasma patterning method with a sacrificial aluminum layer, which shield the oxygen plasma activation only in the aimed position of the substrate. This approach is more robust than the conventional «plasma deactivation» approach based on the chemical patterning via micro-contact printing. For the valve regulation, no external control line is necessary but a weak pressure to the injection channel. Although our valve has no simpler structure than that of the hydrophobic valve which has ever been the simplest, our valve has a significant advantage in the repeatable utility. Due to its simplicity, moreover, long term injection more than 120 min was easily and precisely carried out. These highest simplicity and performance are quite suitable for the large integration and mass production of complicated lab-on-a-chip system for the application to biological research.