Insulation resistance detecting apparatus

An insulation resistance detecting apparatus which detects insulation resistance between a DC power supply and a ground of an external unit including said DC power supply, comprising:

• a pulse generator that generates a rectangular pulse that have a constant period and a constant duty ratio;
• a comparator to which a reference voltage is supplied as one input and an output from said pulse generator is supplied as the other input, said comparator outputting a signal of a first level when a level of said other input exceeds said reference voltage, and outputting a signal of a second level different from said first level when a level of said other input is lower than said reference voltage;
• a first resistance provided in series on an output line of said pulse generator;
• a coupling capacitor one end of which is connected to a line by which said first resistance is connected to said other input of said comparator and the other end of which is connected to an output line of said DC power supply;
• a capacitor one end of which is connected to a portion on said line, said portion being located between said first resistance and said coupling capacitor, said line being connected to said other input of said comparator, and the other end of which is connected to said ground;
• and
• a pulse width measuring device that measures a pulse width of an output waveform of said comparator to calculate a duty ratio of said output waveform and that calculates a value of said insulation resistance on the basis of said calculated duty ratio.