We propose to use the speed of ultrasonic to measure the absolute values of gas concentration. This method can detect the gas concentration using the sound speed of difference of each gas and calculate the absolute values of its concentration. It is possible to measure it without calibration and inexpensively using this method. In this study, we demonstrated oxygen and hydrogen concentration measurements in nitrogen base gas. The error of oxygen concentration measurement was about 2.0 % in the temperature varying condition. On the other hand the error of hydrogen measurement was about 0.1% even in the temperature variation from -10 to 50°C.