A plane coordinate controller for converting length position into electric potential that is able to be received by computer has a circular thin plate made of resistance material as its main body, on whose edge three cylindrical electrodes are installed. The circle centers of three electrodes are positioned at apexes of an isosceles right triangle. When a DC voltage is applied between electrodes, the constant-current field on circular plate is mathematically and physically analyzed based on electrostatic analogy to create the function relationship between the potential value at any point on circular plate and coordinate of the same point in plane coordinate system.