We present passive flow-rate regulators using an autonomous deflection of parallel membrane valves, capable to maintain a constant flow-rate at varying inlet pressure supplied from micropumps. The previous passive flow-rate regulators are difficult to integrate with micropumps, not only because of the complex multi-layer structures, but also because of the high threshold inlet pressure required for flow-rate regulation. In this study, we present passive flow-rate regulators using parallel membrane valves, capable of achieving flow-rate regulation function at the minimum threshold inlet pressure as low as 15 kPa with simple structure formed by a single mask process. The parallel membranes in a flow-rate regulator are designed to deflect and adjust flow resistance autonomously according to the inlet pressure, thus maintaining a constant flow-rate independent of the inlet pressure variation. We designed the four different prototypes of W20, W30, W40, and W50, having parallel membrane widths of 20, 30, 40 and 50 mu m, respectively. We estimated the flow-rate based on both analytical and numerical models. In an experimental study, we observed the deformation of parallel membranes and the flow-rate depending on the inlet pressure. The fabricated prototypes achieved the constant flow-rate of 6.09 +/- 0.32 mu l s(-1) (W20 fabricated by 10 : 1 PDMS (PolyDiMethylSiloxane)) over an inlet pressure of 20 kPa. We also observed that prototypes fabricated by 20 : 1 PDMS, having lower Young's modulus than normal 10 : 1 PDMS, showed a lower threshold pressure and higher regulated flow-rate than prototypes fabricated by 10 : 1 PDMS. W40 fabricated by 20 : 1 PDMS showed a constant flow-rate of 14.53 +/- 0.51 mu l s(-1) over inlet pressure of 15 kPa. The present passive flow-rate regulators have strong potential for applications in integrated microfluidic systems.