As the amount of urea removed from the blood (MB) is equal to that transferred into the spent dialysate during dialysis treatment, MB can be estimated from the urea concentration in the spent dialysate and the flow rate of the dialysate. There has been a need to develop a sensor for measuring the urea concentration during dialysis treatment instead of using reagents to detect urea in the blood. We have created a prototype of a precise urea sensor for measuring urea concentration in the spent dialysate using chemiluminescence (CL). We have reported an accuracy of 3% for a CL-based urea sensor system with a syringe-pump-type CL reactor and a photosensor module (PM) as the CL detector. However, problems including the poor mechanical durability of the large reactor unit and the high cost of the CL detector with a PM must still be solved. A compact and low-cost urea sensor that can be installed in a dialysis machine is needed. We have attempted to increase the CL emission so as to measure CL using a low-cost photodiode (PD) and to make a reactor that is highly durable. The urea sensor based on CL due to the reaction of urea with hypobromite has a high precision of 1.9%. It also exhibits good correlation with the chemically measured urea concentrations in Okayama Medical Laboratories, Inc. The CL emitted from the new vortex-flow-type reactor was detected using a low-cost Si PD (S1787-08 Hamamatsu, Japan) and a high-gain current amplifier (7 mV/pA), so that we can measure the urea nitrogen concentrations between 2 and 30 mg/dL. This sensor will be used to realize automatic dialysis machines in the near future.