Disclosed is a device for sensing the opening degree of a valve, which can easily determine the upper and lower limits without using upper and lower limit switches so that, besides simplifying the mechanical structure, the device can be easily installed, maintained, and repaired. The device can avoid basic errors and improve the opening degree sensing performance without using upper and lower limit switches, cams, and a spline so that, by simplifying the mechanical structure, the device can be manufactured, installed, maintained, and repaired at a low cost, and its long-term durability is improved. The device has a main gear adapted to disengage from the meter gear when the main gear is rotated excessively so that any damage to the meter gear and potentiometer, which may occur during installation or use, is prevented.