A microfluidic lactic acid (LA) monitoring system for sweat LA was fabricated and tested. Sweat is one of the promising candidate for non-invasive monitoring of chemical content in human body. However, there are several difficulty in sweat content monitoring resulted from sweating mechanism. We employed a continuous flow of phosphate buffer saline (PBS) at the surface of the skin as a career flow for transporting whole secretions to a biosensor, which was highly specific to LA. The advantage of using continuous flow as sweat career is the capability of continuous monitoring. Because the surface of the skin is always refreshed by the continuous flow, initial condition of the skin surface is maintained throughout measurement. The sampling device was improved using a medical plaster. This resulted in tight attachment to the skin's surface and reduction of leakage so that LA monitoring was capable for highly active subjects. In the experiment, the sampling device was connected to the biosensor, which sensitivity to LA was 53.1 nA μM-1. The biosensor was operated using a wireless potentiostat and the continuous career flow was supplied by dual diaphragm pump system. As a real-sample test, the monitoring system was utilized to LA monitoring of jogging subject and swimming subject, respectively. In both cases, changes of LA corresponding to exercises were successfully monitored from the free-moving subject and the swimming subject for the first time.