A fast responce hydrogen sensor using ultrasonic was developed and demonstrated. It uses the difference in sound velocity between hydrogen and the air. Thus, it is possible to measure hydrogen concentration by measuring the sound velocity changing when hydrogen is mixed in the air. The hydrogen sensor using ultrasonic in this study can be very fast in response time which was less than 84 msec. We also have shown that it is possible to detect hydrogen concentration as low as 100 ppm with the distance between ultrasonic probes as small as 20 mm. Temperature variation test was also demonstrated between -10 and 50°C and confirmed that it can match with calculation results.