Disclosed is a transformer for a gas insulated electrical apparatus which requires no grounding field and which provides high accuracy with a simple structure. A transformer includes a capacitor type voltage divider having a floating electrode at a position opposed to a high-voltage conductor through an insulation sealing terminal mounted to a flange of a branch pipe provided on an inner surface of the annular grounding container; an ohm resistor for voltage division which has an impedance that is sufficiently smaller than the impedance between the floating electrode and the annular grounding container; and a converting portion which converts the detection voltage of the high-voltage conductor to a voltage value that allows processing on the secondary side.