1. A plating analyzing method for analyzing an electroplating system having an anode, a cathode and plating liquid, based on a Laplace's equation, the method comprising: making the Laplace's equation discrete by a Finite Volume Method including dividing the electroplating system into a plurality of three-dimensional elements and giving values for unknown potentials of the three-dimensional elements to the Laplace's equation; forming equations based on the discrete Laplace's equation; and calculating potential distribution using the equations, wherein said making the Laplace's equation discrete comprises: making a first two-dimensional element face of a three-dimensional element coincide with an anode surface or a cathode surface; taking a difference between a first potential in an anode side or cathode side element having the first two-dimensional element face as its boundary and a second potential in a plating liquid side element having the first two-dimensional element face as its boundary, as a polarization voltage, said cathode surface including a conducting region in which an electric current flows from the plating liquid and a non-conducting region in which no electric current flows; making a second two-dimensional element face coincide with the conducting region or the non-conducting region; designating a third two-dimensional element face to include the conducting region and the non-conducting region; and determining a current density flowing through the third two-dimensional element face based on a current density of an electric current flowing through the conducting region and a ratio of an area of the conducting region to an entire area of the third two-dimensional element face which is designated to include the conducting region and the non-conducting region.