We propose to use the speed of sound for measuring an absolute value of hydrogen concentration in the air or in a gas. Since the speed of sound of hydrogen is much faster than other atmospheric gases, detection of the change in the speed of sound leads to the measurement of an absolute value of hydrogen concentration when hydrogen is mixed in the gas. In this study we showed that it is possible to measure the hydrogen concentration as low as 100ppm in the air. We also performed the experiment under various temperatures conditions for practical use and showed that it is possible to compensate for the temperature by calculation. Therefore, this ultrasonic measuring method can be also used as a hydrogen sensor.