A polarity discriminating method and signal processing circuit for a vapor sensor in a microwave oven is disclosed. The control section operates a fan motor so as to discriminate the polarity of a detecting signal supplied from the vapor sensor, which is varied with the wind produced by the fan motor. After the control section initializes variables of counters, the control section compares the magnitude of the signal-processed detecting signal supplied from the detecting signal processing circuit section with the magnitude of a reference detecting signal in a specified range on the phase coordinate axis. Meanwhile, the control section discriminates whether the slope of the curve of the signal-processed detecting signal is positive or negative, thereby discriminating whether the vapor sensor operates in a positive polarity mode or negative polarity mode. Furthermore, the configuration of the signal processing circuit for the vapor sensor in the microwave oven is simplified, so that the unit cost required for manufacturing the circuit decreases and the volume of the circuit is reduced. Therefore, the malfunction of the electronic circuit section installed in the microwave oven is prevented.