NOVELTY - The method involves measuring a first value of an electric current, which flows between an output terminal (50a) and a grounding part in a measuring object body i.e. photovoltaic power generation unit (50), when a voltage for inspection is applied between the terminal and the part. A second value of the electric current, which flows between the terminal and the part is measured when the voltage is stopped between the terminal and the part. An insulation resistance value based on a third value, which is deducted from the two values and a voltage value is calculated.    USE - Method for measuring an insulation resistance of a photovoltaic power generation unit i.e. solar cell module. Can also be used for a wind power generation unit, a hydroelectric generating unit, a tidal-power-generation unit and a geothermal energy unit.    ADVANTAGE - The insulation resistance value between the terminal and the part is calculated based on the electric current value, which is deducted from the another two electric current values and the voltage value without short-circuiting a pair of output terminals, so that an operation to cover the photovoltaic power generation unit installed in a high place with a shielding cloth is avoided, thus avoiding irradiation of sunlight to detect a measurement during night time, and hence deducting the value of the electric current resulting from an electrical potential difference produced among the output terminals and measuring the insulation resistance value of the photovoltaic power generation unit correctly. The first value of the electric current, which flows between the output terminal and the grounding part is calculated based on each changed voltage value and each measured electric current value by a co-linear approximation process, so that the insulation resistance value of the measuring object body can be measured more correctly.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a method for inspecting an insulation state of a solar cell module    (2) an insulation resistance measuring apparatus comprising a control part.    DESCRIPTION OF DRAWING(S) - The drawing shows a circuit block diagram of an insulation test apparatus.'(Drawing includes non-English language text)'    Power supply part (2)    Measurement part (3)    Control part (8)    Photovoltaic power generation unit (50)    Output terminals (50a, 50b)