The concentration of metal debris in the lubricating oil system can be utilized to indicate the condition of the machinery directly. Oil monitoring techniques via inductive sensors have been widely used to predict mechanical failures. The proposed online inducive debris sensor in this work uses a built-in planar coil as an energizing coil to produce a radial magnetic field, which provides a high throughput and sensitive debris-detecting equipment. The sensing coil was optimally designed via simulation of the magnetic field characteristics by the planar energizing coils. The postprocessing circuit system was developed to purify debris signals. A test platform established in this article was efficiently utilized to evaluate the performance of the sensor. Experimental results have demonstrated that the designed sensor can successfully detect 152- $\mu$ m ferric debris and 467- $\mu \text{m}$ nonferric debris in a 30-mm-diameter tube.