Microfluidic washing techniques that are essential to many microchip-based bioassays are still premature for practical applications. Here, we propose a passive washing protocol for microchip-based bioassay applications; this approach uses the inlet-pressure difference between two solution inlets, and it employs a washing valve that regulates the merging of two solutions. The inlet-pressure difference is mainly induced by capillarity. Sample and washing solutions merge at the washing valve, and the passive washing process occurs just after the merging. This passive washing protocol does not require any mechanical valves as well as mechanical pumps for microfluidic control. The washing volume and direction are regulated by controlling the volume of solutions injected to the inlets. In addition, the washing speed can be programmed by controlling the microchannel width. As a model bioassay, the binding of biotin-4-fluorescein inside a reaction chamber is quantified by using the passive washing. The passive washing protocol provides a simple but effective microfluidic washing method.