A diagnosis device and method of a voltage transformer can include a current transformer disposed at one side of an output port for generating a current variation according to an on/off operation of a power switch element; a first current detector configured to detect a forward direction current from the current transformer; a second current detector configured to detect a reverse direction current from the current transformer; a first voltage transformer configured to transform the forward direction current detected from the first current detector to a forward direction voltage; a second voltage transformer configured to transform the reverse direction current detected from the second current detector to a reverse direction voltage; and a diagnosis portion for comparing the forward direction voltage with the reverse direction voltage to determine whether or not an offset is formed.