An inverter circuit of a microwave oven employs a technique which decreases the unit manufacturing cost of production and controls an output of a magnetron correctly. The invert circuit includes a micom outputting a reference voltage preset by a user; an input current detector converting a current provided from the power source to a voltage level; and comparator comparing an output signal of the input current detector with the reference voltage; an integrator increasing or decreasing a level of an output signal for the comparator a trigger circuit connected to the high voltage transformer, and generating a trigger signal; and a PWM controller receiving an output signal of the integrator and an output al of the trigger circuit, for controlling drive of the magnetron.