Provided is an automatic sensor-calibration method in which a state change of a sensor, caused by aging characteristics or contamination of the sensor, is sensed by a sensor input unit to adjust, according to the sensed state change of the sensor, a slice level used as a criterion for a light receiving device to determine whether a medium exists, or the slice level is adjusted according to the sensed state change of the sensor and a current level of a light emitting device is adjusted when the slice level reaches a preset absolute slice level, or when contamination of the sensor is removed, the current level of the light emitting device is reduced to a minimum step, the slice level is set, and then the current level of the light emitting device is adjusted based on the set slice level, thereby preventing malfunction of the sensor.