PURPOSE: To detect deterioration in a porcelain tube accurately, by subtracting a leakage current for a capacitance portion determined from an applied voltage and an electrostatic capacitance of a bushing in a vector manner from a measuring current flowing through the porcelain tube to calculate a leakage current for a resistance portion.

CONSTITUTION: A bushing 1 is made up of a conductor 3 inserted through a porcelain tube 2. Then, an electrode 9 for detecting a leakage current is inserted on the lower voltage side of the porcelain 2 to be connected to an arithmetic device 11, to which 11 are connected a bushing input detector 12 to detect a voltage applied to the bushing 1 and a data memory 13 and a decision device 14 to input an electrostatic capacitance or the like of the bushing 1. The arithmetic device 11 subtracts a leakage current for a capacitance portion, determined from an applied voltage and the electrostatic capacity of the bushing from a measuring a current to calculate a leakage current for a resistance portion. This enables production of a contaminated state monitoring device of the bushing which can detect deterioration in the porcelain tube positively.