This paper reports on a pressure sensor using ionic liquid to measure the pressure of the reflux flowing in the Body-on-a-Chip that mimics human physiological response by a microfluidic device. To design the sensor structure, the sensor output signal dependence of the pressure was estimated. First, deformation of PDMS thin film depending on the applied pressure was analyzed by finite element analysis considering the non-linear material property of a PDMS, and then the resistance change was estimated as the sensor output. Next, the sensor was fabricated by multilayer soft lithography technology, and the resistance change was measured. Comparison between the simulation results and measurements exhibits almost the same trend, revealing the capability to measure a small pressure range of 0 to 12 kPa required for the Body-on-a-Chip device.