An environmental radon monitoring system, comprising a radon-cup, an etching system, and a track counting system, was constructed. The radon cup is a cylindrical chamber with a radius of 2.2 cm and a height of 3.2 cm in combination with a CR-39 detector. Carbon is impregnated in the bodies of the detector chamber to avoid problem of an electrostatic charge. The optimized etching condition for the CR-39 exposed to a radon environment turned out to be a 6 N NaOH solution at 70^{circ}$ over a 7hour period. The bulk etch rate under the optimized condition was $1.14{pm}0.03;{mu}m;h^{-1}$. The diameter of the tracks caused by radon and its progeny were found to be in the range of $10~25;{mu}m$ under the optimized condition. The track images were observed with a track counting system, which consisted of an optical microscope, a color charged couple device (CCD) camera, and an image processor. The calibration factor of this system is obtained to be $0.105{pm}0.006$ tracks $cm^2$ per Bq $m^{-3}$ d.