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We developed and tested a new underground radon detector using an optical fiber. Previous underground radon detectors used a small-diameter photo-multiplier tube (PMT) behind the chamber, thus, the diameter of the underground radon detector was determined by the size of the PMT. The larger diameter of the detector resulted in considerable labor for drilling holes into soil. The new underground radon detector consists of a small chamber, an optical fiber, and a PMT. The small chamber is a scintillation detector using a ZnS(Ag) film. The optical fiber transfers the scintillated light produced in the chamber to the PMT that is positioned above the soil. In this configuration, the size of the detector was not determined by the size of the PMT. The diameter of the optical fiber used was 5 mm and the outside diameter of the detector was reduced to be 12 mm. Although the light lost from the optical fiber was about 90%, the level of the scintillation signal was much higher than the noise level produced by the PMT and electronics. Measuring the performance of the underground radon detector, we found that the energy response had a clear distribution due to alpha particles emitted by radon and its decay products. The temporal response of the detector was approximately 2 h. Sensitivity was approximately 0.01 counts/h/Bq/m3, one third of the previous underground radon detector. These results indicate the developed radon detector can be used for continuous measurements of radon concentration in underground soil with easy handling.