IN AN ULTRASONIC APPARATUS (13A, 13B) FOR MEASURING THE CONCENTRATION OF GAS PROVIDED WITH TWO ULTRASONIC TRANSDUCERS (2A, 2B) FOR TRANSMITTING/ RECEIVING AN ULTRASONIC WAVE, ARRANGED OPPOSITE TO EACH OTHER IN PIPING (1) THROUGH WHICH A SAMPLE GAS (M) FLOWS, A TEMPERATURE SENSOR (3), AND A PRESSURE SENSOR (8), AN ULTRASONIC METHOD AND APPARATUS (13A, 13B) THAT CAN ACCURATELY MEASURE GAS CONCENTRATION REGARDLESS OF PRESSURE OF THE SAMPLE GAS (M) ARE PROVIDED AS AN ULTRASONIC APPARATUS (13A, 13B) FOR MEASURING THE CONCENTRATION OF GAS COMPRISING CONCENTRATION CALCULATING MEANS FOR CALCULATING SAMPLE GAS CONCENTRATION BASED ON A PROPAGATION SPEED CORRECTION COEFFICIENT BY PRESSURE.