Abstract: A method for continuously measuring and monitoring insulation resistance values of a non-grounded DC circuit in an operative condition and an apparatus for practicing the method are disclosed. This method is to calculate various values including the values of the insulation resistances across three lines forming the non-grounded DC circuit by continuously measuring only two voltages across the three lines forming that circuit. The apparatus includes voltage detecting means, switching means, memory means, and calculating means. A load resistance is connected to each of the positive power line and the negative power line, and the switching means operates to selectively ground these load resistances. The voltage detecting means further includes a buffer circuit. The memory means temporarily stores the detected voltage value. The calculating means is connected to the memory means for calculating various values including the insulation resistance values utilizing the voltage values.